Method for manufacturing mold

ABSTRACT

A method for manufacturing a mold, including steps of (a) expanding a powder mixture containing a water-soluble inorganic salt powder and a water-soluble organic polymer powder in a plane on a table and then selectively spraying a spray liquid containing at least one selected from a water-miscible organic solvent and water through a nozzle on this powder mixture layer to make a bonding strength among particles of the powder mixture in a sprayed region higher than that among particles of the powder mixture in an unsprayed region, (b) further expanding the powder mixture on the powder mixture layer after spraying and then selectively spraying the spray liquid through the nozzle on this powder mixture layer, (c) repeatedly conducting the step (b) over several times to form a mold shaped by the sprayed regions, and (d) taking out the mold.

PRIORITY

This application is a U.S. national phase application under 35 USC 371of International Application PCT/JP2006/325293 (not published inEnglish), filed Dec. 19, 2006, which claims priority from 2005-375575filed in Japan on Dec. 27, 2005, both references of which areincorporated in their entireties.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing a mold, andin particular to a method for manufacturing a mold by a laminatedmold-making technique in which a mold such as a core used in productionof a casting mold is automatically produced on the basis of computerdata.

BACKGROUND OF THE INVENTION

This kind of laminated mold-making technique is a method ofmanufacturing a mold such as a core from computer data, for example CADdata, wherein a mold of complicated shape can be produced in a simpleprocess.

Jpn. Pat. Appln. KOKAI Publication No. 2004-82206 discloses a method formanufacturing a mold by utilizing a laminated mold-making technique,wherein a water-soluble powder (for example, a salt powder) expanded ina plane is printed (sprayed through a nozzle) with a liquid (forexample, an aqueous solution containing an adhesive substance such asgum arabic or starch) on the basis of CAD data by a three-dimensionalprinter having a nozzle, to make the bonding strength among particles ofthe salt powder in the sprayed region higher than that among CLEANVERSION particles of the salt powder in the unsprayed region, and thenfurther expanding the salt powder thereon and similarly spraying theaqueous solution containing an adhesive substance thereon through thenozzle, is conducted repeatedly a desired number of times. This processis advantageous in that because the resulting mold is made of awater-soluble salt powder, the mold can be easily dissolved (removed)with water after casting.

However, the spray liquid contains an adhesive substance (for example,gum arabic, starch, etc.) and is highly viscous, so a nozzle may beclogged with the spray liquid when sprayed several times from athree-dimensional printer having a nozzle in producing a mold. When theviscosity of the spray liquid is reduced by decreasing the amount of theadhesive substance therein in order to prevent the clogging of thenozzle, the bonding strength among particles of the salt powder in thesprayed region may be reduced to cause collapse of the mold uponremoval.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a method formanufacturing a mold, wherein a mold of a desired shape with sufficientstrength can be produced reproducibly without nozzle clogging uponspraying with a spray liquid through a nozzle and can be easily removed.

Another object of the present invention is to provide a method formanufacturing a mold, wherein a mold of a desired shape with a smoothsurface and higher strength can be produced reproducibly without nozzleclogging upon spraying with a spray liquid through a nozzle and can beeasily removed.

According to a first aspect of the present invention, there is provideda method for manufacturing a mold, comprising steps of (a) expanding apowder mixture containing a water-soluble inorganic salt powder and awater-soluble organic polymer powder in a plane on a table and thenselectively spraying a spray liquid containing at least one selectedfrom a water-miscible organic solvent and water through a nozzle on thispowder mixture layer to make a bonding strength among particles of thepowder mixture in a sprayed region higher than that among particles ofthe powder mixture in an unsprayed region; (b) further expanding thepowder mixture on the powder mixture layer after spraying and thenselectively spraying the spray liquid through the nozzle on this powdermixture layer; (c) repeatedly conducting the step (b) over several timesto form a mold shaped by the sprayed regions; and (d) taking out themold.

According to a second aspect of the present invention, there is provideda method for manufacturing a mold, comprising steps of (a) expanding apowder mixture containing a water-soluble first inorganic salt powder, asecond inorganic salt powder consisting of magnesium sulfate, and awater-soluble organic polymer powder in a plane on a table and thenselectively spraying a spray liquid containing at least one selectedfrom a water-miscible organic solvent and water through a nozzle on thispowder mixture layer to make a bonding strength among particles of thepowder mixture in a sprayed region higher than that among particles ofthe powder mixture in an unsprayed region; (b) further expanding thepowder mixture on the powder mixture layer after spraying and thenselectively spraying the spray liquid through the nozzle on this powdermixture layer; (c) repeatedly conducting the step (b) over several timesto form a mold shaped by the sprayed regions; (d) taking out the mold;and (e) heating the mold at a temperature between the temperature atwhich the water-soluble organic polymer is burned out and thetemperature at which the first and second inorganic salts are not fused.

According to a third aspect of the present invention, there is provideda method for manufacturing a mold, comprising steps of (a) expanding apowder mixture containing a water-soluble first inorganic salt water, awater-soluble second inorganic salt powder having a melting point lowerthan that of the first inorganic salt, and a water-soluble organicpolymer powder in a plane on a table and then selectively spraying aspray liquid containing at least one selected from a water-miscibleorganic solvent and water through a nozzle on this powder mixture layerto make a bonding strength among particles of the powder mixture in asprayed region higher than that among particles of the powder mixture inan unsprayed region; (b) further expanding the powder mixture on thepowder mixture layer after spraying and then selectively spraying thespray liquid through the nozzle on this powder mixture layer; (c)repeatedly conducting the step (b) over several times to form a moldshaped by the sprayed regions; (d) taking out the mold; and (e) heatingthe mold at a temperature at which the first inorganic salt is not fusedbut the second inorganic salt is fused.

According to a fourth aspect of the present invention, there is provideda method for manufacturing a mold, comprising steps of (a) expanding apowder mixture containing a water-soluble first inorganic salt powderand a water-soluble organic polymer powder in a plane on a table andthen selectively spraying a spray liquid containing a second inorganicsalt having a melting point lower than that of the first inorganic salt,and either water or water and a water-miscible organic solvent through anozzle on this powder mixture layer to make a bonding strength amongparticles of the powder mixture in a sprayed region higher than thatamong particles of the powder mixture in an unsprayed region; (b)further expanding the powder mixture on the powder mixture layer afterspraying and then selectively spraying the spray liquid through thenozzle on this powder mixture layer; (c) repeatedly conducting the step(b) over several times to form a mold shaped by the sprayed regions; (d)taking out the mold; and (e) heating the mold at a temperature at whichthe first inorganic salt is not fused but the second inorganic salt isfused.

According to a fifth aspect of the present invention, there is provideda method for manufacturing a mold, comprising steps of (a) expanding apowder mixture containing a water-soluble first inorganic salt powderand an organic polymer powder in a plane on a table and then selectivelyspraying a spray liquid containing at least one second inorganic salthaving a melting point lower than that of the first inorganic salt andselected from calcium chloride and sodium iodide and at least oneorganic solvent selected from a lower alcohol and a ketone through anozzle on this powder mixture layer to make a bonding strength amongparticles of the powder mixture in a sprayed region higher than thatamong particles of the powder mixture in an unsprayed region; (b)further expanding the powder mixture on the powder mixture layer afterspraying and then selectively spraying the spray liquid through thenozzle on this powder mixture layer; (c) repeatedly conducting the step(b) over several times to form a mold shaped by the sprayed regions; (d)taking out the mold; and (e) heating the mold at a temperature at whichthe first inorganic salt is not fused but the second inorganic salt isfused.

According to a sixth aspect of the present invention, there is provideda method for manufacturing a mold, comprising steps of (a) expanding apowder mixture containing a water-soluble first inorganic salt powderand a water-soluble organic polymer powder in a plane on a table andthen selectively spraying a spray liquid containing at least oneselected from a water-miscible organic solvent and water through anozzle on this powder mixture layer to make a bonding strength amongparticles of the powder mixture in a sprayed region higher than thatamong particles of the powder mixture in an unsprayed region; (b)further expanding the powder mixture on the powder mixture layer afterspraying and then selectively spraying the spray liquid through thenozzle on this powder mixture layer; (c) repeatedly conducting the step(b) over several times to form a mold shaped by the sprayed regions; (d)taking out the mold; and (e) further spraying a treatment compositioncontaining a water-soluble second inorganic salt having a melting pointlower than that of the first inorganic salt and at least one liquidselected from water and a water-miscible organic solvent on a surface ofthe mold, or dipping the mold in the treatment composition, and thenheating the mold after either treatment at a temperature at which thefirst inorganic salt is not fused but the second inorganic salt isfused.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a method for manufacturinga mold according to a first embodiment of the invention.

FIG. 2 is a graph showing the strengths of molds produced in Examples 1to 7 and Comparative Examples 1 and 2 according to the invention.

DETAILED DESCRIPTION

Hereinafter, the process for producing a mold according to the presentinvention will be described in detail.

First Embodiment First Step

A powder mixture containing a water-soluble inorganic salt powder and awater-soluble organic polymer powder is expanded in a plane on a table.Then, the expanded powder mixture layer is sprayed selectively through anozzle with a spray liquid containing at least one selected from awater-miscible organic solvent and water. At this time, the organicpolymer powder acting as a binder is contained in the powder mixture,thus making the bonding strength among particles of the powder mixturein the sprayed region higher than that among particles of the powdermixture in the unsprayed region.

The water-soluble inorganic salt powder is contained as the maincomponent in the powder mixture, that is, in an amount exceeding 50% byweight, particularly preferably in an amount of 70% by weight or more.

The water-soluble inorganic salt can be used, for example, an alkalimetal salt and an alkaline earth metal salt. The alkali metal salt isfor example an alkali metal halide selected from sodium chloride,potassium chloride, sodium iodide and sodium bromide. The alkaline earthmetal salt is for example an alkaline earth metal halide selected fromcalcium chloride and magnesium chloride. These inorganic salts can beused in the form of a mixture of two or more thereof.

The water-soluble inorganic salt powder desirably has an averageparticle size of 350 μm or less, more preferably 30 to 150 μm.

As the water-soluble organic polymer, various kinds of organic polymersmay be used. An example of such an organic polymer is at least oneselected from cellulose derivatives such as polyvinyl pyrrolidone (PVP),polyvinyl alcohol (PVA), polyethylene glycol (PEG), methyl cellulose,ethyl cellulose, ethylhydroxymethyl cellulose, carboxymethyl cellulose(CMC), hydroxypropyl cellulose (HPC) and hydroxypropylmethyl cellulose(HPMC), gum arabic, gelatin, starch and flour. Among these water-solubleorganic polymers, PVP, PVA and PEG are preferable because they havedispersibility to prevent agglomeration of the coexisting water-solubleinorganic salt powder, have characteristics to improve fluidity, and canimprove the ability of the powder mixture to expanded in a plane.Particularly, PVP is preferable because it is most excellent in aneffect of improving the dispersibility and fluidity of the inorganicsalt powder.

The water-soluble organic polymer powder desirably has an averageparticle size of 350 μm or less, more preferably 30 to 150 μm. Thewater-soluble organic polymer powder having an average particle sizelarger than that of the inorganic salt powder is preferably selected,because of improving the ability of the powder mixture to be uniformlymixed.

The content of the water-soluble organic polymer powder in the powdermixture is preferably 0.1 to 30% by weight. When the content of thewater-soluble organic polymer powder is less than 0.1% by weight, theshape of the resulting mold cannot be maintained, and the resulting moldmay easily collapse. On the other hand, when the content of thewater-soluble organic polymer powder exceeds 30% by weight, theresulting mold has an increased amount of the water-soluble organicpolymer, and when used as a core for casting, may reduce the heatresistance relatively and simultaneously increase the degree ofshrinkage to make it difficult to obtain a high-precision casting. Thecontent of the water-soluble organic polymer powder is more preferably0.5 to 15% by weight.

The water-miscible organic solvent contained in the spray liquid has afunction to suppress or prevent blurring and diffusion of the sprayedregion when the expanded powder mixture layer is sprayed through anozzle with the spray liquid. Such water-miscible organic solvent can beused, for example, lower alcohols such as ethanol, methanol andpropanol, ketones such as acetone, methyl ethyl ketone, methyl propylketone and isopropyl methyl ketone, and alkyl acetates such as methylacetate and ethyl acetate. Among these organic solvents, lower alcoholsand acetone are particularly preferable.

When the spray liquid contains a water-miscible organic solvent andwater, their mixing ratio (water-miscible organic solvent to water) ispreferably 5:95 to 75:25 by weight.

The spray liquid may further contain a dye. By spraying the expandedpowder mixture layer through a nozzle with a spray liquid containingsuch a dye, the sprayed region is colored, and therefore, a) the abilityof the sprayed region to be discriminated from the unsprayed region canbe improved, b) spraying through a nozzle can be visually observed in aresting state and thus malfunction of the nozzle can be found early, c)the workability in removing the unsprayed region to remove a mold can beimproved, and d) the dimensional accuracy of a mold taken out can beimproved. A dye may also similarly be incorporated into the sprayliquids in the second to fifth embodiments described later.

The spray liquid is used usually at room temperature, but may be heated.For example, when the spray liquid is composed exclusively of water, thespray liquid is preferably heated at 60□ at maximum. When the sprayliquid consists of a mixture of a water-miscible organic solvent andwater, the spray liquid is heated preferably up to a temperature lowerby about 10□ than the boiling point of the water-miscible organicsolvent. For example, when the water-miscible organic solvent isethanol, the spray liquid is preferably heated at 60□ at maximum. Whenthe water-miscible organic solvent is acetone, the spray liquid ispreferably heated at 40□ at maximum.

Second Step

The powder mixture is further expanded on the powder mixture layer afterspraying. Then, the spray liquid is sprayed selectively through thenozzle on this expanded powder mixture layer.

Third Step

The second step is conducted repeatedly several times to form a moldshaped by the sprayed regions formed by spraying the spray liquidthrough the nozzle several times.

In making a laminated mold in the first to third steps, spraying of thespray liquid through a nozzle can be carried out for example byconverting data obtained by three-dimensional CAD into data on a sectionsliced in a desired thickness, printing the expanded powder mixturelayer with the spray liquid by a three-dimensional printer having anozzle, on the basis of the bottom section data, as described above, andprinting similarly repeatedly expanded powder mixture layers with thespray liquid respectively on the basis of the data on sections frombottom to top by the three-dimensional printer.

Fourth Step

After the third step, a mold is manufactured by removing the powdermixture in the unsprayed region and taking out the mold.

The production process according to such first embodiment will bedescribed specifically with reference to (A) to (E) of FIG. 1.

As shown in (A) of FIG. 1, a powder mixture containing a water-solubleinorganic salt powder and a water-soluble organic polymer powder isfirst expanded in a plane on a table 1 to form a powder mixture layer 2₁. As shown in (B) of FIG. 1, the powder mixture layer 2 ₁ is thenprinted (sprayed through a nozzle) with a spray liquid containing atleast one selected from a water-miscible organic solvent and water by athree-dimensional printer 3 having a nozzle, on the basis of the bottom(data on the lowest section) among the section data described above. Atthis time, the bonding strength among particles of the powder mixture ina sprayed region 4 a in the powder mixture layer 2 ₁ is made higher thanthat among particles of the powder mixture in an unsprayed region 4 b.

As shown in (C) of FIG. 1, the powder mixture is further expanded on thepowder mixture layer 2 ₁ after spraying as described above, to form apowder mixture layer 22. As shown in (D) of FIG. 1, this expanded powdermixture layer 22 is then printed (sprayed through a nozzle) with thespray liquid by the three-dimensional printer 3 having a nozzle, on thebasis of the section data (data on an upper section next to the bottom).At this time, the bonding strength among particles of the powder mixturein a sprayed region 5 a in the powder mixture layer 22 is made higherthan that among particles of the powder mixture in an unsprayed region 5b. Simultaneously, the sprayed region 5 a is bound to the sprayed region4 a just below the sprayed region 5 a.

Then, the steps of (C) and (D) of FIG. 1 described above are repeatedlycarried out. In this process, the respective expanded powder mixturelayers are printed (sprayed through a nozzle) in series with the sprayliquid by the three-dimensional printer having a nozzle, on the basis ofthe section data (data on the third section and thereafter) describedabove. In such laminated mold-making, a mold (for example, a pyramidalmold 6) shaped by the sprayed regions formed by spraying the sprayliquid several times is produced as shown in (E) of FIG. 1. Thereafter,the powder mixture in the unsprayed region is removed to take out themold. In this manner, the pyramidal mold is manufactured.

After manufacture of a laminated mold in the first embodiment describedabove, the mold before taking out may be dried at a temperature of 200□or less, and the mold after taking out may be dried at 200□ or less.

In the first embodiment, the resulting mold may be heat-treated in anoxygen-containing atmosphere, for example in an air atmosphere, at atemperature between the temperature at which the water-soluble organicpolymer is burned out (degreased) and the temperature at which theinorganic salt is not fused.

According to the first embodiment described above, the following actionand effect are demonstrated.

1) A powder mixture containing a water-soluble inorganic salt powder anda water-soluble organic polymer powder is expanded in a plane on atable, and then the expanded powder mixture layer is sprayed selectivelythrough a nozzle (for example, by printing with a three-dimensionalprinter having a nozzle) with a spray liquid containing at least oneselected from a water-miscible organic solvent and water, whereby all ora part of the water-soluble organic polymer powder as a component of thepowder mixture can be dissolved with the spray liquid, thus renderingthe bonding strength among particles of the powder mixture in thesprayed region higher than that among particles of the powder mixture inthe unsprayed region. Such expanding of the powder mixture and selectivespraying with the spray liquid through a nozzle are repeated severaltimes, whereby a mold of a desired shape shaped by the sprayed regionscan be formed.

2) The water-soluble organic polymer powder incorporated into the powdermixture acts as a powder binder in the sprayed region, while the sprayliquid is a composition containing at least one selected from awater-miscible organic solvent and water and being free of a bindercomponent involved substantially in increasing viscosity, therebypreventing the clogging of a nozzle upon spraying the spray liquid ontothe expanded powder mixture layer (by printing with, for example, athree-dimensional printer having a nozzle). As a result, the sprayedregions (slice regions for forming a mold) faithful to the section datainto which the three-dimensional CAD data has been sliced can be formedin laminated mold manufacturing. In addition, there is no limitation,attributable to clogging, of the number of laminates in the laminatedmold thus produced, and thus a larger mold can be manufactured.

Particularly, by using a spray liquid containing a water-miscibleorganic solvent, it is possible to suppress or prevent the blurring anddiffusion of the sprayed region upon spraying the expanded powdermixture layer selectively through a nozzle with the spray liquid, thusenabling formation of sprayed regions (slice regions for forming a mold)faithful to the section data into which the three-dimensional CAD datahas been sliced.

3) In demolding after casting with the mold as a core, the mold iscomposed mainly of a water-soluble inorganic salt and a water-solubleorganic polymer and can thus be easily removed with water.

According to the first embodiment, therefore, there can be provided amethod for manufacturing a mold, wherein a mold of a desired shape withsufficient strength can be produced reproducibly without clogging anozzle in spraying a spray liquid and can be easily removed aftercasting.

Second Embodiment First Step

A powder mixture containing a water-soluble first inorganic salt powder,a second inorganic salt powder consisting of magnesium sulfate, and awater-soluble organic polymer powder is expanded in a plane on a table.Then, the expanded powder mixture layer is sprayed selectively through anozzle with a spray liquid containing at least one selected from awater-miscible organic solvent and water. At this time, the organicpolymer powder acting as a binder is contained in the powder mixture,thus making the bonding strength among particles of the powder mixturein the sprayed region higher than that among particles of the powdermixture in the unsprayed region.

The water-soluble first inorganic salt powder and the second inorganicsalt powder consisting of magnesium sulfate are contained as the maincomponents in the powder mixture, that is, in an amount exceeding 50% byweight, particularly preferably in an amount of 70% by weight or more.

The first inorganic salt that can be used, for example, alkali metalsalts such as sodium chloride and potassium chloride and alkaline earthmetal salts (excluding magnesium sulfate) such as calcium chloride,calcium sulfate and magnesium chloride. Particularly, the firstinorganic salt is preferably sodium chloride.

The first and second inorganic salt powders desirably have an averageparticle size of 350 μm or less, more preferably 30 to 150 μm.

The second inorganic salt powder is compounded in an amount of 50% byweight or less based on the first inorganic salt powder. Particularly,the first inorganic salt powder and the second inorganic salt powder arecompounded such that the weight ratio of the first inorganic salt powderto second inorganic salt powder is from 99.5:0.5 to 50:50, morepreferably from 99.5:0.5 to 90:10.

The water-soluble organic polymer used may be the same as describedabove in the first embodiment. Among the water-soluble organic polymers,PVP, PVA and PEG have dispersibility to prevent agglomeration of thecoexisting water-soluble first and second inorganic salt powders, andhave characteristics to improve fluidity and can improve the ability ofthe powder mixture to expand in a plane. The water-soluble organicpolymer powder desirably has an average particle size of 350 μm or less,more preferably 30 to 150 μm. The water-soluble organic polymer powderhaving an average particle size larger than that of the inorganic saltpowder is preferably selected, because of improving the ability of thepowder mixture to be uniformly mixed.

The content of the water-soluble organic polymer powder in the powdermixture is desirably as described in the first embodiment, that is, 0.1to 30% by weight, more preferably 0.5 to 15% by weight.

The water-miscible organic solvent contained in the spray liquid has afunction to suppress or prevent blurring and diffusion of the sprayedregion when the expanded powder mixture layer is sprayed through anozzle with the spray liquid, as described in the first embodiment. Suchwater-miscible organic solvent can be used, for example, lower alcoholssuch as ethanol, methanol and propanol, ketones such as acetone, methylethyl ketone, methyl propyl ketone and isopropyl methyl ketone, andalkyl acetates such as methyl acetate and ethyl acetate. Among theseorganic solvents, lower alcohols and acetone are particularlypreferable.

When the spray liquid contains a water-miscible organic solvent andwater, their mixing ratio (water-miscible organic solvent to water) ispreferably 5:95 to 75:25 by weight.

The spray liquid is used usually at room temperature, but may be heated.For example, when the spray liquid consists exclusively of water, thespray liquid is preferably heated at 60□ at maximum. When the sprayliquid consists of a mixture of a water-miscible organic solvent andwater, the spray liquid is heated preferably up to a temperature lowerby about 10□ than the boiling point of the water-miscible organicsolvent. For example, when the water-miscible organic solvent isethanol, the spray liquid is preferably heated at 60□ at maximum. Whenthe water-miscible organic solvent is acetone, the spray liquid ispreferably heated at 40□ at maximum.

Second Step

The powder mixture is further expanded on the powder mixture layer afterspraying. Then, the spray liquid is sprayed selectively through thenozzle on this expanded powder mixture layer.

Third Step

The second step is conducted repeatedly several times to form a moldshaped by the sprayed regions formed by spraying the spray liquidthrough the nozzle several times.

In making a laminated mold in the first to third steps, the spray liquidis sprayed through a nozzle in the same manner as described in the firstembodiment.

Fourth Step

After the third step, a mold is produced by removing the powder mixturein the unsprayed region and taking out the mold.

Fifth Step

The final mold is produced by heating the resulting mold in anoxygen-containing atmosphere, for example in an air atmosphere, at atemperature between the temperature at which the water-soluble organicpolymer is burned out and the temperature at which the first and secondinorganic salts are not fused.

The heating is carried out preferably at 400 to 700□ in anoxygen-containing atmosphere, for example in an air atmosphere, forexample in the case of a combination wherein the first inorganic salt issodium chloride and the second inorganic salt is magnesium sulfate.

Production of a laminated mold in the method for manufacturing a moldaccording to the second embodiment is carried out specifically in thesame manner as in the steps of (A) to (E) of FIG. 1 described above inthe first embodiment except that the powder mixture used is acomposition containing the water-soluble first inorganic salt powder,the second inorganic salt powder consisting of magnesium sulfate, andthe water-soluble organic polymer powder.

After manufacture of a laminated mold in the second embodiment describedabove, the mold before taking out may be dried at a temperature of 200□or less, and the mold after taking out may be dried at 200□ or less.

In the second embodiment, the surface of the mold after the fifth stepmay be sprayed with a treatment composition containing a third inorganicsalt having a melting point lower than that of the first inorganic saltand at least one liquid selected from water and a water-miscible organicsolvent, or the mold may be dipped in the treatment composition, and themold after either treatment may be heat-treated at a temperature atwhich the first and second inorganic salts are not fused but the thirdinorganic salt is fused.

The treatment composition is mainly in the form of a solution. Theconcentration of the third inorganic salt in this treatment compositionis preferably in the range of 0.1 to 25% by weight. However, thetreatment composition consisting of the third inorganic salt combinedwith the water-miscible organic solvent is sometimes in the form of aslurry in which the third inorganic salt is suspended withoutdissolution in the water-miscible organic solvent, depending on theirtypes.

The third inorganic salt is selected from different kinds of materialsfrom the first and second inorganic salt. For example, when the firstinorganic salt is NaCl, the third inorganic salt can be used KCl, NaI orCaCl₂.

In dipping the mold in the treatment composition, the treatmentcomposition on the surface of the mold is preferably 20 to 50 μm inthickness.

By fusing the third inorganic salt in the heat treatment describedabove, it is possible to further improve the strength of the mold, inaddition to an improvement in the surface smoothness of the mold.

When the first inorganic salt in the mold is, for example, sodiumchloride (the second inorganic salt is magnesium sulfate) and the thirdinorganic salt in the treatment composition is potassium chloride, theheating is conducted preferably at a temperature of 400 to 700□ in anoxygen-containing atmosphere, for example in an air atmosphere. In thecase of a combination wherein the first inorganic salt is NaCl (thesecond inorganic salt is magnesium sulfate) and the third inorganic saltis NaI, the heating is conducted preferably at a temperature of 400 to700□ in an oxygen-containing atmosphere, for example in an airatmosphere.

According to the second embodiment described above, the following actionand effect are demonstrated.

1) A powder mixture containing a water-soluble first inorganic saltpowder, a second inorganic salt powder consisting of magnesium sulfate,and a water-soluble organic polymer powder is expanded in a plane on atable, and then the expanded powder mixture layer is sprayed selectivelythrough a nozzle with a spray liquid containing at least one selectedfrom a water-miscible organic solvent and water (for example, byprinting with a three-dimensional printer having a nozzle), whereby allor a part of the water-soluble organic polymer powder as a component ofthe powder mixture can be dissolved with the spray liquid, thusrendering the bonding strength among particles of the powder mixture inthe sprayed region higher than that among particles of the powdermixture in the unsprayed region. Such expanding of the powder mixtureand selective spraying of the spray liquid through a nozzle are repeatedseveral times, whereby a mold of a desired shape shaped by the sprayedregions can be formed.

2) The water-soluble organic polymer powder incorporated into the powdermixture acts as a powder binder in the sprayed region, while the sprayliquid is a composition containing at least one selected from awater-miscible organic solvent and water and being free of a bindercomponent involved substantially in increasing viscosity, therebypreventing the clogging of a nozzle upon spraying the spray liquid ontothe expanded powder mixture layer (by printing with, for example, athree-dimensional printer having a nozzle). As a result, the sprayedregions (slice regions for forming a mold) faithful to the section datainto which the three-dimensional CAD data has been sliced can be formedin laminated mold manufacturing. In addition, there is no limitation,attributable to clogging, of the number of laminates in the laminatedmold produced, and thus a larger mold can be manufactured.

Particularly, by using a spray liquid containing a water-miscibleorganic solvent, it is possible to suppress or prevent the blurring anddiffusion of the sprayed region upon spraying the expanded powdermixture layer through a nozzle with the spray liquid, thus enablingformation of sprayed regions (slice regions for forming a mold) faithfulto the section data into which the three-dimensional CAD data has beensliced.

3) The mold after taking out has a desired sharp shape by usingmagnesium sulfate as the second inorganic salt.

4) The mold after taking out is heated at a temperature between thetemperature at which the water-soluble organic polymer is burned out andthe temperature at which the first and second inorganic salts are notfused, whereby the water-soluble organic polymer can be removed.

5) In removal of the mold as a core after casting, the mold is composedmainly of the water-soluble first and second inorganic salts, so themold can be easily removed with water.

According to the second embodiment, there can be provided a method formanufacturing a mold, wherein a mold having a smooth and glossy surfaceand practically durable strength and being faithful to three-dimensionalCAD data, can be manufactured reproducibly without clogging a nozzle inspraying the spray liquid, and can be easily removed.

According to the second embodiment, the water-soluble organic polymer isburned out by heating, and thus the resulting mold has a slightly lowerstrength than that of the mold having the remaining water-solubleorganic polymer according to the first embodiment, but has the followingeffect.

That is, when the mold having the remaining water-soluble organicpolymer is applied to a core where a molten metal is cast at atemperature higher than the temperature at which the water-solubleorganic polymer is burned out, for example, to a core for aluminumdie-casting, a gas is generated when the water-soluble organic polymeris burned out. This gas generation causes die-casting to generatebubbles, which reduce the strength, resulting in a defective product.Accordingly, the water-soluble organic polymer in the mold is burned outby the heating, thereby preventing gas generation attributable to thewater-soluble organic polymer. As a result, bubble generation, etc. inthe die-casting product, accompanying the gas generation, can beprevented, and the desired die-casting product can be obtained.

The surface of the mold after the heating is further sprayed with atreatment composition containing a third inorganic salt having a meltingpoint lower than that of the first inorganic salt and at least oneliquid selected from water and a water-miscible organic solvent, or themold is dipped in the treatment composition, and the mold after eithertreatment is heated at a temperature at which the first and secondinorganic salts are not fused but the third inorganic salt is fused. Bycoating the surface of the mold with the third inorganic salt in thetreatment composition and subsequent heating, the surface of the mold issmoothened by coating the surface with a thin film of the thirdinorganic salt. In addition, the mold is densified by fusing the thirdinorganic salt.

When such a mold having a smooth surface is applied to a core foraluminum die-casting, gas generation attributable to the water-solubleorganic polymer can be avoided, thereby preventing bubble generation inthe die-casting product, as described above, and the contact surface ofthe die-casting product with the mold (core) can be made glossy, andthus a high-grade die-casting product having the desired strength can beobtained.

Third Embodiment First Step

A powder mixture containing a water-soluble first inorganic salt powder,a water-soluble second inorganic salt powder having a melting pointlower than the first inorganic salt powder, and a water-soluble organicpolymer powder is expanded in a plane on a table. Then, the expandedpowder mixture layer is sprayed selectively through a nozzle with aspray liquid containing at least one selected from a water-miscibleorganic solvent and water. At this time, the organic polymer powderacting as a binder is contained in the powder mixture, thus making thebonding strength among particles of the powder mixture in the sprayedregion higher than that among particles of the powder mixture in theunsprayed region.

The water-soluble first and second inorganic salt powders are containedas the main components in the powder mixture, that is, in an amountexceeding 50% by weight, particularly preferably in an amount of 70% byweight or more.

The first inorganic salt and second inorganic salt may be, for example,an alkali metal salt or an alkaline earth metal salt, and can be used,for example, in any of the following combinations:

(1) a combination wherein the first inorganic salt is sodium chloride,and second inorganic salt is potassium chloride,

(2) a combination wherein the first inorganic salt is sodium chloride,and second inorganic salt is calcium chloride,

(3) a combination wherein the first inorganic salt is sodium chloride,and second inorganic salt is magnesium chloride, and

(4) a combination wherein the first inorganic salt is sodium chloride,and second inorganic salt is calcium sulfate.

The first and second inorganic salt powders desirably have an averageparticle size of 350 μm or less, more preferably 30 to 150 μm.

The first inorganic salt powder is compounded preferably in an amountequal to, or more than, the amount of the second inorganic salt powder.Specifically, the first inorganic salt powder and the second inorganicsalt powder are preferably compounded such that the weight ratio of thefirst inorganic salt powder to second inorganic salt powder is from99.5:0.5 to 50:50.

The water-soluble organic polymer used may be the same as describedabove in the first embodiment. Among the water-soluble organic polymers,PVP, PVA and PEG have dispersibility to prevent agglomeration of thecoexisting water-soluble first and second inorganic salt powders, havecharacteristics to improve fluidity and can improve the ability of thepowder mixture to expand in a plane. The water-soluble organic polymerpowder desirably has an average particle size of 350 μm or less, morepreferably 30 to 150 μm. The water-soluble organic polymer powder havingan average particle size larger than that of the inorganic salt powderis preferably selected, because of improving the ability of the powdermixture to be uniformly mixed.

The content of the water-soluble organic polymer powder in the powdermixture is desirably as described in the first embodiment, that is, 0.1to 30% by weight, more preferably 0.5 to 15% by weight.

The water-miscible organic solvent contained in the spray liquid has afunction to suppress or prevent blurring and diffusion of a sprayedregion upon spraying the spray liquid through a nozzle onto the expandedpowder mixture layer, as described in the first embodiment. Suchwater-miscible organic solvent that can be used includes, for example,lower alcohols such as ethanol, methanol and propanol, ketones such asacetone, methyl ethyl ketone, methyl propyl ketone and isopropyl methylketone, and alkyl acetates such as methyl acetate and ethyl acetate.Among these organic solvents, lower alcohols and acetone areparticularly preferable.

When the spray liquid contains a water-miscible organic solvent andwater, their mixing ratio (water-miscible organic solvent to water) ispreferably 5:95 to 75:25 by weight.

The spray liquid is used usually at room temperature, but may be heated.For example, when the spray liquid consists exclusively of water, thespray liquid is preferably heated at 60□ at maximum. When the sprayliquid consists of a mixture of a water-miscible organic solvent andwater, the spray liquid is heated preferably up to a temperature lowerby about 10□ than the boiling point of the water-miscible organicsolvent. For example, when the water-miscible organic solvent isethanol, the spray liquid is preferably heated at 60□ at maximum. Whenthe water-miscible organic solvent is acetone, the spray liquid ispreferably heated at 40□ at maximum.

Second Step

The powder mixture is further expanded on the powder mixture layer afterspraying. Then, the spray liquid is sprayed selectively through thenozzle on this expanded powder mixture layer.

Third Step

The second step is conducted repeatedly several times to form a moldshaped by the sprayed regions formed by spraying the spray liquidthrough the nozzle several times.

In making a laminated mold in the first to third steps, the spray liquidis sprayed through a nozzle in the same manner as described in the firstembodiment.

Fourth Step

After the third step, a mold is manufactured by removing the powdermixture in the unsprayed region and taking out the mold.

Fifth Step

The resulting mold is heated in an oxygen-containing atmosphere, forexample in an air atmosphere, at a temperature at which the firstinorganic salt is not fused but the second inorganic salt is fused,whereby the final mold is manufactured. At this time, the water-solubleorganic polymer in the mold is burned out (degreased).

In the case of the combination where the first inorganic salt is sodiumchloride and the second inorganic salt is potassium chloride, theheating is conducted preferably at a temperature of 400 to 750□ in anoxygen-containing atmosphere, for example in an air atmosphere.

Production of a laminated mold in the method for manufacturing a moldaccording to the third embodiment is carried out specifically in thesame manner as in the steps of (A) to (E) of FIG. 1 described above inthe first embodiment except that the powder mixture used is acomposition containing the water-soluble first inorganic salt powder,the water-soluble second inorganic salt powder having a melting pointlower than that of the first inorganic salt, and the water-solubleorganic polymer powder.

After manufacture of a laminated mold in the third embodiment describedabove, the mold before taking out may be dried at a temperature of 200□or less, and the mold after taking out may be dried at 200□ or less.

In the third embodiment, the surface of the mold after the fifth stepmay be sprayed with a treatment composition containing a third inorganicsalt having a melting point lower than that of the first inorganic saltand at least one liquid selected from water and a water-miscible organicsolvent, or the mold may be dipped in the treatment composition, and themold after either treatment may be heated at a temperature at which thefirst inorganic salt is not fused but at least one inorganic saltselected from the second and third inorganic salts is fused.

The treatment composition is mainly in the form of a solution. Theconcentration of the third inorganic salt in this treatment compositionis preferably in the range of 0.1 to 25% by weight. However, thetreatment composition consisting of the third inorganic salt combinedwith the water-miscible organic solvent is sometimes in the form of aslurry in which the third inorganic salt is suspended withoutdissolution in the water-miscible organic solvent, depending on theirtypes.

The third inorganic salt is selected from materials of the same type as,or of a different type from, that of the second inorganic salt. When thethird inorganic salt is a material of a different type from the firstand second inorganic salts, the combination of the first, second andthird inorganic salts include, for example, a combination wherein thefirst inorganic salt is NaCl, the second inorganic salt is KCl, and thethird inorganic salt is NaI or CaCl₂.

In dipping the mold in the treatment composition, the treatmentcomposition on the surface of the mold is preferably 20 to 50 μm inthickness.

The “fusion” of at least one inorganic salt selected from the secondinorganic salt and the third inorganic salt in the heating means thatwhen the second and third inorganic salts are of the same type, theinorganic salts are simply fused. When the third inorganic salt is amaterial of a type different from the second inorganic salt, the term“fusion” means (A) mere fusion of the second inorganic salt and/or thethird inorganic salt, (B) formation of a solid solution of all of thesecond and third inorganic salts upon fusion, (C) formation of a solidsolution of some of the second and third inorganic salts andsimultaneous formation of a composite salt between the remaininginorganic salts upon fusion, and (D) formation of a composite salt ofall the second and third inorganic salts upon fusion. By fusing at leastone inorganic salt selected from the second and third inorganic salts inthis way, it is possible to further improve the strength of the mold, inaddition to an improvement in the surface smoothness of the mold.

When the third inorganic salt in the treatment composition is potassiumchloride in a combination wherein the first inorganic salt in the moldis, for example, sodium chloride and the second inorganic salt ispotassium chloride, the heating is conducted preferably at a temperatureof 400 to 750□ in an oxygen-containing atmosphere, for example in an airatmosphere. In the case of a combination wherein the first inorganicsalt is NaCl, the second inorganic salt is KCl, and the third inorganicsalt is NaI, the heating is conducted preferably at a temperature of 400to 700□ in an oxygen-containing atmosphere, for example, in an airatmosphere.

According to the third embodiment described above, the following actionand effect are demonstrated.

1) A powder mixture containing a water-soluble first inorganic saltpowder, a water-soluble second inorganic salt powder having a meltingpoint lower than the first inorganic salt powder, and a water-solubleorganic polymer powder is expanded in a plane on a table, and then theexpanded powder mixture layer is sprayed selectively through a nozzlewith a spray liquid containing at least one selected from awater-miscible organic solvent and water (for example, by printing witha three-dimensional printer having a nozzle), whereby all or a part ofthe water-soluble organic polymer powder as a component of the powdermixture can be dissolved with the spray liquid, thus rendering thebonding strength among particles of the powder mixture in the sprayedregion higher than that among particles of the powder mixture in theunsprayed region. Such expanding of the powder mixture and selectivespraying of the spray liquid through a nozzle are repeated severaltimes, whereby a mold of a desired shape shaped by the sprayed regionscan be formed.

2) The water-soluble organic polymer powder incorporated into the powdermixture acts as a powder binder in the sprayed region, while the sprayliquid is a composition containing at least one selected from awater-miscible organic solvent and water and being free of a bindercomponent involved substantially in increasing viscosity, therebypreventing the clogging of a nozzle upon spraying the spray liquid ontothe expanded powder mixture layer (by printing with, for example, athree-dimensional printer having a nozzle). As a result, the sprayedregions (slice regions for forming a mold) faithful to the section datainto which the three-dimensional CAD data has been sliced can be formedin laminated mold manufacturing. In addition, there is no limitation,attributable to clogging, of the number of laminates in the laminatedmold thus produced, and thus a larger mold can be manufactured.

Particularly, by using a spray liquid containing a water-miscibleorganic solvent, it is possible to suppress or prevent the blurring anddiffusion of the sprayed region upon spraying the expanded powdermixture layer selectively through a nozzle with the spray liquid, thusenabling formation of sprayed regions (slice regions for forming a mold)faithful to the section data into which the three-dimensional CAD datahas been sliced.

3) The mold after taking out is heated at a temperature at which thefirst inorganic salt is not fused but the second inorganic salt isfused, whereby the fused second inorganic salt permeates into minutevoids in the first inorganic salt powder to make the mold dense. Byfusing the second inorganic salt on the surface of the mold, the surfacecan be smoothened. As a result, a mold having a glossier surface thanthe mold just after taking out can be obtained.

4) In demolding after casting with the mold as a core, the mold iscomposed mainly of the water-soluble first and second inorganic saltsand can thus be easily removed with water.

According to the third embodiment, there can be provided a method formanufacturing a mold, wherein a mold having a smooth and glossy surfaceand practically durable strength and being faithful to three-dimensionalCAD data can be produced reproducibly without clogging a nozzle inspraying the spray liquid and can be easily removed.

According to the third embodiment, the water-soluble organic polymer isburned out by heating the mold at a temperature at which the secondinorganic salt is fused, and thus the resulting mold has a slightlylower strength than that of the mold having the remaining water-solubleorganic polymer according to the first embodiment, but has the followingeffect.

That is, when the mold having the remaining water-soluble organicpolymer is applied to a core where a molten metal is cast at atemperature higher than the temperature at which the water-solubleorganic polymer is burned out, for example, to a core for aluminumdie-casting, a gas is generated when the water-soluble organic polymeris burned out. This gas generation causes die-casting to generatebubbles, which reduce strength, resulting in a defective product.Accordingly, the water-soluble organic polymer in the mold is burned outby the heating, thereby preventing gas generation attributable to thewater-soluble organic polymer. As a result, bubble generation, etc. inthe die-casting product, accompanying the gas generation, can beprevented, and the die-casting product having a desired strength can beobtained.

The surface of the mold after the heat treatment is further sprayed witha treatment composition containing a third inorganic salt having amelting point lower than that of the first inorganic salt and at leastone liquid selected from water and a water-miscible organic solvent, orthe mold is dipped in the treatment composition, and the mold aftereither treatment is heat-treated at a temperature at which the firstinorganic salt is not fused but at least one inorganic salt selectedfrom the second and third inorganic salts is fused. By coating thesurface of the mold with the third inorganic salt in the treatmentcomposition and subsequent heating, the surface of the mold issmoothened by coating it with a thin film of the third inorganic salt.In addition, the mold is densified by fusing at least one of the secondand third inorganic salts. Particularly, both the second and thirdinorganic salts can be fused to form a mutual solid solution or to forma composite salt, thus further densifying the mold and improving thestrength thereof. As a result, a high-strength mold having a smootherand glossier surface can be obtained.

When such a mold having a smooth surface is applied, for example, to acore for aluminum die-casting, gas generation attributable to thewater-soluble organic polymer can be avoided, thereby preventing bubblegeneration in the die-casting product, as described above, and thecontact surface of the die-casting product with the mold (core) can bemade glossy, and thus a high-grade die-casting product having a desiredstrength can be obtained.

Fourth Embodiment First Step

A powder mixture containing a water-soluble first inorganic salt powderand a water-soluble organic polymer powder is expanded in a plane on atable. Then, the expanded powder mixture layer is sprayed selectivelythrough a nozzle with a spray liquid containing a water-soluble secondinorganic salt having a melting point lower than that of the firstinorganic salt and either water or a mixture consisting of water and awater-miscible organic solvent. At this time, the organic polymer powderacting as a binder is contained in the powder mixture, thus making thebonding strength among particles of the powder mixture in the sprayedregion higher than that among particles of the powder mixture in theunsprayed region.

The water-soluble first inorganic salt powder is contained as the maincomponent in the powder mixture, that is, in an amount exceeding 50% byweight, particularly preferably in an amount of 70% by weight or more.

The water-soluble first inorganic salt in the powder mixture and thesecond inorganic salt in the spray liquid are used in, for example, thecombinations (1) to (4) described above in the third embodiment.

The first inorganic salt powder desirably has an average particle sizeof 350 μm or less, more preferably 30 to 150 μm.

The water-soluble organic polymer used may be the same as describedabove in the first embodiment. Among the water-soluble organic polymers,PVP, PVA and PEG have dispersibility to prevent agglomeration of thecoexisting water-soluble first inorganic salt powder, havecharacteristics to improve fluidity and can improve the ability of thepowder mixture to expand in a plane. The water-soluble organic polymerpowder desirably has an average particle size of 350 μm or less, morepreferably 30 to 150 μm. The water-soluble organic polymer powder havingan average particle size larger than that of the inorganic salt powderis preferably selected, because of improving the ability of the powdermixture to be uniformly mixed.

The content of the water-soluble organic polymer powder in the powdermixture is desirably as described in the first embodiment, that is, 0.1to 30% by weight, more preferably 0.5 to 15% by weight.

The water-miscible organic solvent contained in the spray liquid has afunction to suppress or prevent blurring and diffusion of the sprayedregion when the expanded powder mixture layer is sprayed through anozzle with the spray liquid, as described in the first embodiment. Suchwater-miscible organic solvents that can be used include, for example,lower alcohols such as ethanol, methanol and propanol, ketones such asacetone, methyl ethyl ketone, methyl propyl ketone and isopropyl methylketone, and alkyl acetates such as methyl acetate and ethyl acetate.Among these organic solvents, lower alcohols and acetone areparticularly preferable.

When the spray liquid contains a water-miscible organic solvent andwater, their mixing ratio (water-miscible organic solvent to water) ispreferably 5:95 to 75:25 by weight.

The concentration of the second inorganic salt in the spray liquid ispreferably 0.1% by weight or more, from the viewpoint of improving thestrength of a mold by the second inorganic salt. The upper limit of theconcentration of the second inorganic salt in the spray liquid ispreferably the maximum concentration of the second inorganic salt thatcan be dissolved at room temperature in at least one selected from awater-miscible organic solvent and water. Particularly, theconcentration of the second inorganic salt in the spray liquid ispreferably 0.1 to 25% by weight.

The spray liquid is used usually at room temperature, but may be heated.For example, when the spray liquid consists of the second inorganic saltand water, the spray liquid is preferably heated at 60□ at maximum. Whenthe spray liquid consists of a mixture of the second inorganic salt, thewater-miscible organic solvent and water, the spray liquid is heatedpreferably up to a temperature lower by about 10□ than the boiling pointof the water-miscible organic solvent. For example, when thewater-miscible organic solvent is ethanol, the spray liquid ispreferably heated at 60□ at maximum. When the water-miscible organicsolvent is acetone, the spray liquid is preferably heated at 40□ atmaximum.

Second Step

The powder mixture is further expanded on the powder mixture layer afterspraying. Then, the spray liquid is sprayed selectively through thenozzle on this expanded powder mixture layer.

Third Step

The second step is conducted repeatedly several times to form a moldshaped by the sprayed regions formed by spraying the spray liquidseveral times.

In making a laminated mold in the first to third steps, the spray liquidis sprayed through a nozzle in the same manner as described in the firstembodiment.

Fourth Step

After the third step, a mold is produced by removing the powder mixturein the unsprayed region and taking out the mold.

Fifth Step

The resulting mold is heated in an oxygen-containing atmosphere, forexample in an air atmosphere, at a temperature at which the firstinorganic salt is not fused but the second inorganic salt is fused,whereby the final mold is produced. At this time, the water-solubleorganic polymer in the mold is burned out (degreased).

In the case of the combination where the first inorganic salt is sodiumchloride and the second inorganic salt is potassium chloride, theheating is conducted preferably at a temperature of 400 to 750□ in anoxygen-containing atmosphere, for example in an air atmosphere.

Production of a laminated mold in the method for manufacturing a moldaccording to the fourth embodiment is carried out specifically in thesame manner as in the steps of (A) to (E) of FIG. 1 described above inthe first embodiment except that the powder mixture used contains awater-soluble first inorganic salt powder and a water-soluble organicpolymer powder, and the sprayed liquid used contains a second inorganicsalt to be fused with the first inorganic salt by heating and eitherwater or a water-miscible organic solvent and water.

After manufacture of a laminated mold in the fourth embodiment describedabove, the mold before taking out may be dried at a temperature of 200□or less, and the mold after taking out may be dried at 200□ or less.

In the fourth embodiment, the surface of the mold after the fifth stepmay be sprayed with a treatment composition containing a third inorganicsalt having a melting point lower than that of the first inorganic saltand at least one liquid selected from water and a water-miscible organicsolvent, or the mold may be dipped in the treatment composition, and themold after either treatment may be heated at a temperature at which thefirst inorganic salt is not fused but at least one inorganic saltselected from the second and third inorganic salts is fused.

The treatment composition is mainly in the form of a solution. Theconcentration of the third inorganic salt in this treatment compositionis preferably in the range of 0.1 to 25% by weight. However, thetreatment composition consisting of the third inorganic salt combinedwith the water-miscible organic solvent is sometimes in the form of aslurry in which the third inorganic salt is suspended withoutdissolution in the water-miscible organic solvent, depending on theirtypes.

The third inorganic salt is selected from materials of the same type as,or of a different type from, that of the second inorganic salt. When thethird inorganic salt is a material of a different type from the firstand second inorganic salts, the combination of the first, second andthird inorganic salts includes, for example, a combination wherein thefirst inorganic salt is sodium chloride (NaCl), the second inorganicsalt is potassium chloride (KCl), and the third inorganic salt is sodiumiodide (NaI) or calcium chloride (CaCl₂).

In dipping the mold into the treatment composition, the treatmentcomposition on the surface of the mold is preferably 20 to 50 μm inthickness.

The “fusion” of at least one inorganic salt selected from the secondinorganic salt and the third inorganic salt in the heating treatmentmeans that when the second and third inorganic salts are of the sametype, the inorganic salts are simply fused. When the third inorganicsalt is a material of a type different from the second inorganic salt,the term “fusion” means (A) mere fusion of the second inorganic saltand/or the third inorganic salt, (B) formation of a solid solution ofall of the second and third inorganic salts upon fusion, (C) formationof a solid solution of some of the second and third inorganic salts andsimultaneous formation of a composite salt between the remaininginorganic salts upon fusion, and (D) formation of a composite salt ofall the second and third inorganic salts upon fusion. By fusing at leastone inorganic salt selected from the second and third inorganic salts inthis way, it is possible to further improve the strength of the mold, inaddition to an improvement in the surface smoothness of the mold.

When the third inorganic salt in the treatment composition is potassiumchloride (KCl) in a combination wherein the first inorganic salt in themold is for example sodium chloride (NaCl) and the second inorganic saltis potassium chloride (KCl), the heating is conducted preferably at atemperature of 400 to 750□ in an oxygen-containing atmosphere, forexample in an air atmosphere. In the case of a combination wherein thefirst inorganic salt is sodium chloride (NaCl), the second inorganicsalt is potassium chloride (KCl), and the third inorganic salt is sodiumiodide (NaI), the heat treatment is conducted preferably at atemperature of 400 to 700□ in an oxygen-containing atmosphere, forexample in an air atmosphere.

According to the fourth embodiment described above, the following actionand effect are demonstrated.

1) A powder mixture containing a water-soluble first inorganic saltpowder and a water-soluble organic polymer powder is expanded in a planeon a table, and then the expanded powder mixture layer is sprayedselectively through a nozzle (for example, by printing with athree-dimensional printer having a nozzle) with a spray liquidcontaining a water-soluble second inorganic salt having a melting pointlower than that of the first inorganic salt and either water or amixture consisting of water and a water-miscible organic solvent,whereby all or a part of the water-soluble organic polymer powder as acomponent of the powder mixture can be dissolved with the spray liquid,thus rendering the bonding strength among particles of the powdermixture in the sprayed region higher than that among particles of thepowder mixture in the unsprayed region. Such expanding of the powdermixture and selective spraying with the spray liquid through a nozzleare repeated several times, whereby a mold of a desired shape shaped bythe sprayed regions can be formed.

2) The water-soluble organic polymer powder incorporated into the powdermixture acts as a powder binder in the sprayed region, while the sprayliquid is a composition containing the second inorganic salt and eitherwater or a mixture consisting of water and a water-miscible organicsolvent and being free of a binder component involved substantially inincreasing viscosity, thereby preventing the clogging of a nozzle uponspraying the spray liquid onto the expanded powder mixture layer (byprinting with, for example, a three-dimensional printer having anozzle). As a result, the sprayed regions (slice regions for forming amold) faithful to the section data into which the three-dimensional CADdata has been sliced can be formed in laminated mold manufacturing. Inaddition, there is no limitation, attributable to clogging, of thenumber of laminates in the laminated mold thus produced, and thus alarger mold can be manufactured.

Particularly, by using a spray liquid containing a water-miscibleorganic solvent, it is possible to suppress or prevent the blurring anddiffusion of the sprayed region upon spraying the expanded powdermixture layer selectively through a nozzle with the spray liquid, thusenabling formation of sprayed regions (slice regions for forming a mold)faithful to the section data into which the three-dimensional CAD datahas been sliced.

3) The mold after taking out is heat-treated at a temperature at whichthe first inorganic salt is not fused but the second inorganic salt isfused, whereby the fused second inorganic salt permeates into minutevoids in the first inorganic salt powder to make the mold dense. Byfusing the second inorganic salt on the surface of the mold, the surfacecan be smoothened. As a result, a mold having a glossier surface thanthe mold just after removal can be obtained.

4) In demolding after casting with the mold as a core, the mold iscomposed mainly of the water-soluble first and second inorganic saltsand can thus be easily removed with water.

According to the fourth embodiment, there can be provided a method formanufacturing a mold, wherein a mold having a smooth and glossy surfaceand practically durable strength and being faithful to three-dimensionalCAD data can be produced reproducibly without clogging a nozzle inspraying the spray liquid and can be easily removed.

According to the fourth embodiment, the water-soluble organic polymer isburned out by heating at a temperature at which the second inorganicsalt is fused, and thus the resulting mold has a slightly lower strengththan that of the mold having the remaining water-soluble organic polymeraccording to the first embodiment, but has the following effect.

That is, when such a mold wherein the water-soluble organic polymer hasbeen burned out by the heating is applied to a core where a molten metalis cast at a temperature higher than the temperature at which thewater-soluble organic polymer is burned out, for example, to a core foraluminum die-casting, the gas generation attributable to thewater-soluble organic polymer can be prevented, as described in thethird embodiment. As a result, bubble generation, etc. in thedie-casting product, accompanying the gas generation, can be prevented,and the die-casting product having desired strength can be obtained.

The surface of the mold after the heat treatment is sprayed with atreatment composition containing a third inorganic salt having a meltingpoint lower than that of the first inorganic salt and at least oneliquid selected from water and a water-miscible organic solvent, or themold is dipped in the treatment composition, and the mold after eithertreatment is heat-treated at a temperature at which the first inorganicsalt is not fused but at least one inorganic salt selected from thesecond and third inorganic salts is fused. By coating the surface of themold with the third inorganic salt in the treatment composition andsubsequent heat treatment, the surface of the mold is smoothened bycoating it with a thin film of the third inorganic salt. In addition,the mold is densified by fusing at least one of the second and thirdinorganic salts. Particularly, both the second and third inorganic saltscan be fused to form a mutual solid solution or to form a compositesalt, thus further densifying the mold and improving the strengththereof. As a result, a high-strength mold having a smoother andglossier surface can be obtained.

When such a mold having a smooth surface is applied to a core foraluminum die-casting, gas generation attributable to the water-solubleorganic polymer can be avoided, thereby preventing bubble generation inthe die-casting product, as described above, and the contact surface ofthe die-casting product with the mold (core) can be made glossy, andthus a high-grade die-casting product having desired strength can beobtained.

Fifth Embodiment

A powder mixture containing a water-soluble first inorganic salt powderand a water-soluble organic polymer powder is expanded in a plane on atable. Then, the expanded powder mixture layer is sprayed selectivelythrough a nozzle with a spray liquid containing at least one secondinorganic salt having a melting point lower than that of the firstinorganic salt and selected from calcium chloride and sodium iodide andat least one organic solvent selected from a lower alcohol and a ketone.At this time, the organic polymer powder acting as a binder is containedin the powder mixture and is dissolved in a specific organic solvent inthe sprayed region, thus making the bonding strength among particles ofthe powder mixture in the sprayed region higher than that amongparticles of the powder mixture in the unsprayed region.

The water-soluble first inorganic salt powder is contained as the maincomponent in the powder mixture, that is, in an amount exceeding 50% byweight, particularly preferably in an amount of 70% by weight or more.

The first inorganic salt can be used, for example, sodium chloride,sodium bromide, potassium chloride, potassium bromide, magnesiumchloride, magnesium sulfate, and the like.

The first inorganic salt powder desirably has an average particle sizeof 350 μm or less, more preferably 30 to 150 μm.

The organic polymer used may be the same water-soluble organic polymeras described above in the first embodiment. Among the water-solubleorganic polymers, PVP, PVA and PEG have dispersibility to preventagglomeration of the coexisting first inorganic salt powder, havecharacteristics to improve fluidity and can improve the ability of thepowder mixture to expand in a plane. The organic polymer powderdesirably has an average particle size of 350 μm or less, morepreferably 30 to 150 μm. The water-soluble organic polymer powder havingan average particle size larger than that of the inorganic salt powderis preferably selected, because of improving the ability of the powdermixture to be uniformly mixed.

The content of the organic polymer powder in the powder mixture isdesirably as described in the first embodiment, that is, 0.1 to 30% byweight, more preferably 0.5 to 15% by weight.

The second inorganic salt such as sodium iodide contained in the sprayliquid is dissolved in a specific organic solvent such as a loweralcohol and dissolved in water as well.

At least one organic solvent selected from lower alcohols and ketones tobe contained in the spray liquid has a function to suppress or preventblurring and diffusion of the sprayed region when the expanded powdermixture layer is sprayed through a nozzle with the spray liquid. Thelower alcohols include, for example, ethanol, methanol and propanol. Theketones include, for example, acetone, methyl ethyl ketone, methylpropyl ketone and isopropyl methyl ketone. Among these organic solvents,ethanol and acetone are particularly preferable.

The concentration of the second inorganic salt in the spray liquid ispreferably 0.1% by weight or more, from the viewpoint of improving thestrength of a mold by the second inorganic salt. The upper limit of theconcentration of the second inorganic salt in the spray liquid ispreferably the maximum concentration of the second inorganic salt thatcan be dissolved at room temperature in at least one organic solventselected from a lower alcohol and a ketone. Particularly, theconcentration of the second inorganic salt in the spray liquid ispreferably 0.1 to 25% by weight.

Second Step

The powder mixture is further expanded on the powder mixture layer afterspraying. Then, the spray liquid is sprayed selectively through thenozzle on this expanded powder mixture layer.

Third Step

The second step is conducted repeatedly several times to form a moldshaped by the sprayed regions formed by spraying the spray liquidseveral times.

In making a laminated mold in the first to third steps, the spray liquidis sprayed through a nozzle in the same manner as described in the firstembodiment.

Fourth Step

After the third step, a mold is produced by removing the powder mixturein the unsprayed region and taking out the mold.

Fifth Step

The resulting mold is heated in an oxygen-containing atmosphere, forexample in an air atmosphere, at a temperature at which the firstinorganic salt is not fused but the second inorganic salt is fused,whereby the final mold is produced. At this time, the organic polymer inthe mold is burned out (degreased).

In the case of the combination where the first inorganic salt is sodiumchloride (NaCl) and the second inorganic salt is sodium iodide (NaI),the heating is conducted preferably at a temperature of 400 to 700□ inan oxygen-containing atmosphere, for example in an air atmosphere.

Production of a laminated mold in the method for manufacturing a moldaccording to the fifth embodiment is carried out specifically in thesame manner as in the steps of (A) to (E) of FIG. 1 described above inthe first embodiment, except that the powder mixture used contains awater-soluble first inorganic salt powder and a water-soluble organicpolymer powder, and the spray liquid used contains a specific secondinorganic salt to be fused with the first inorganic salt by heating anda specific water-miscible organic solvent.

After manufacture of a laminated mold in the fifth embodiment describedabove, the mold before taking out may be dried at a temperature of 200□or less, and the mold after taking out may be dried at 200□ or less.

In the fifth embodiment, the surface of the mold after the fifth stepmay be sprayed with a treatment composition containing a third inorganicsalt having a melting point lower than that of the first inorganic saltand at least one liquid selected from water and a water-miscible organicsolvent, or the mold may be dipped in the treatment composition, and themold after either treatment may be heated at a temperature at which thefirst inorganic salt is not fused but at least one inorganic saltselected from the second and third inorganic salts is fused.

The treatment composition is mainly in the form of a solution. Theconcentration of the third inorganic salt in this treatment compositionis preferably in the range of 0.1 to 25% by weight. However, thetreatment composition consisting of the third inorganic salt combinedwith the water-miscible organic solvent is sometimes in the form of aslurry in which the third inorganic salt is suspended withoutdissolution in the water-miscible organic solvent, depending on theirtypes.

The third inorganic salt is selected from materials of the same type as,or of a different type from, that of the second inorganic salt. When thethird inorganic salt is a material of a different type from the firstand second inorganic salts, the combination of the first, second andthird inorganic salts includes, for example, a combination wherein thefirst inorganic salt is sodium chloride (NaCl), the second inorganicsalt is sodium iodide (NaI) or calcium chloride (CaCl₂), and the thirdinorganic salt is CaCl₂ or NaI.

In dipping the mold into the treatment composition, the treatmentcomposition on the surface of the mold is preferably 20 to 50 μm inthickness.

The “fusion” of at least one inorganic salt selected from the secondinorganic salt and the third inorganic salt in the heating treatmentmeans that when the second and third inorganic salts are of the sametype, the inorganic salts are simply fused. When the third inorganicsalt is a material of a type different from the second inorganic salt,the term “fusion” means (A) mere fusion of the second inorganic saltand/or the third inorganic salt, (B) formation of a solid solution ofall of the second and third inorganic salts upon fusion, (C) formationof a solid solution of some of the second and third inorganic salts andsimultaneous formation of a composite salt between the remaininginorganic salts upon fusion, and (D) formation of a composite salt ofall the second and third inorganic salts upon fusion. By fusing at leastone inorganic salt selected from the second and third inorganic salts inthis way, it is possible to further improve the strength of the mold, inaddition to an improvement in the surface smoothness of the mold.

When the third inorganic salt in the treatment composition is calciumchloride (CaCl₂) in a combination wherein the first inorganic salt inthe mold is, for example, sodium chloride (NaCl) and the secondinorganic salt is sodium iodide

(NaI), the heating is conducted preferably at a temperature of 400 to750□ in an oxygen-containing atmosphere, for example in an airatmosphere.

According to the fifth embodiment described above, the following actionand effect are demonstrated.

1) A powder mixture containing a water-soluble first inorganic saltpowder and an organic polymer powder is expanded in a plane on a table,and then the expanded powder mixture layer is sprayed selectivelythrough a nozzle with a spray liquid containing a specific secondinorganic salt having a melting point lower than that of the firstinorganic salt and a specific organic solvent such as a lower alcohol(for example, by printing with a three-dimensional printer having anozzle), whereby all or a part of the organic polymer powder as acomponent of the powder mixture can be dissolved with the organicsolvent in the spray liquid, thus rendering the bonding strength amongparticles of the powder mixture in the sprayed region higher than thatamong particles of the powder mixture in the unsprayed region. Suchexpanding of the powder mixture and selective spraying of the sprayliquid through a nozzle are repeated several times, whereby a mold of adesired shape shaped by the sprayed regions can be formed.

2) The organic polymer powder incorporated into the powder mixture actsas a powder binder in the sprayed region, while the spray liquid is acomposition containing a specific second inorganic salt and an organicsolvent and being free of a binder component involved substantially inincreasing viscosity, thereby preventing the clogging of a nozzle uponspraying the spray liquid onto the expanded powder mixture layer (byprinting with, for example, a three-dimensional printer having anozzle). As a result, the sprayed regions (slice regions for forming amold) faithful to the section data into which the three-dimensional CADdata has been sliced can be formed in laminated mold manufacturing. Inaddition, there is no limitation, attributable to clogging, of thenumber of laminates in the laminated mold thus produced, and thus alarger mold can be manufactured.

3) By using a spray liquid containing a specific organic solvent, it ispossible to suppress or prevent the blurring and diffusion of thesprayed region upon spraying the expanded powder mixture layer through anozzle with the spray liquid, thus enabling formation of sprayed regions(slice regions for forming a mold) faithful to the section data intowhich the three-dimensional CAD data has been sliced.

4) The mold after taking out is heated at a temperature at which thefirst inorganic salt is not fused but the second inorganic salt isfused, whereby the fused second inorganic salt permeates into minutevoids in the first inorganic salt powder to make the mold dense. Byfusing the second inorganic salt on the surface of the mold, the surfacecan be smoothened. As a result, a mold having a glossier surface thanthe mold just after removal can be obtained.

5) In demolding after casting with the mold as a core, the mold iscomposed mainly of the water-soluble first inorganic salt and thewater-soluble second inorganic salt such as calcium chloride or sodiumiodide and can thus be easily removed with water.

According to the fifth embodiment, there can be provided a method formanufacturing a mold, wherein a mold having a smooth and glossy surfaceand practically durable strength and being faithful to three-dimensionalCAD data can be produced reproducibly without clogging a nozzle inspraying the spray liquid and can be easily removed.

According to the fifth embodiment, the organic polymer is burned out byheating the mold at a temperature at which the second inorganic salt isfused, and thus the resulting mold has a slightly lower strength thanthat of the mold having the remaining water-soluble organic polymeraccording to the first embodiment, but has the following effect.

That is, when such a mold wherein the water-soluble organic polymer hasbeen burned out by the heating is applied to a core where a molten metalis cast at a temperature higher than the temperature at which thewater-soluble organic polymer is burned out, for example, to a core foraluminum die-casting, the gas generation attributable to thewater-soluble organic polymer can be prevented, as described in thethird embodiment. As a result, bubble generation, etc. in thedie-casting product, accompanying the gas generation, can be prevented,and a die-casting product having the desired strength can be obtained.

The surface of the mold after the heating is sprayed with a treatmentcomposition containing a third inorganic salt having a melting pointlower than that of the first inorganic salt and at least one liquidselected from water and a water-miscible organic solvent, or the mold isdipped in the treatment composition, and the mold after either treatmentis heat-treated at a temperature at which the first inorganic salt isnot fused but at least one inorganic salt selected from the second andthird inorganic salts is fused. By coating the surface of the mold withthe third inorganic salt in the treatment composition and subsequentheating, the surface of the mold is smoothened by coating it with a thinfilm of the third inorganic salt. In addition, the mold is densified byfusing at least one selected from the second and third inorganic salts.Particularly, both the second and third inorganic salts can be fused toform a mutual solid solution or to form a composite salt, thus furtherdensifying the mold and improving the strength thereof. As a result, ahigh-strength mold having a smoother and glossier surface can beobtained.

When such a mold having a smooth surface is applied to a core foraluminum die-casting, gas generation attributable to the organic polymercan be avoided, thereby preventing bubble generation in the die-castingproduct, as described above, and the contact surface of the die-castingproduct with the mold (core) can be made glossy, and thus a high-gradedie-casting product having the desired strength can be obtained.

Sixth Embodiment

A laminated mold is manufactured and the mold is removed in the samefirst to fourth steps as in the first embodiment. In these steps,however, the water-soluble first inorganic salt powder is used as thewater-soluble inorganic salt powder in the powder mixture. Thewater-soluble first inorganic salt powder is contained as the maincomponent in the powder mixture, that is, in an amount exceeding 50% byweight, particularly preferably in an amount of 70% by weight or more.The spray liquid is used usually at room temperature, but may be heated.For example, when the spray liquid consists of the second inorganic saltand water, the spray liquid is heated preferably at 60□ at maximum. Whenthe spray liquid consists of the second inorganic salt, a water-miscibleorganic solvent and water, the spray liquid is heated preferably up to atemperature lower by about 10□ than the boiling point of thewater-miscible organic solvent. For example, when the water-miscibleorganic solvent is ethanol, the spray liquid is preferably heated at 60□at maximum. When the water-miscible organic solvent is acetone, thespray liquid is preferably heated at 40□ at maximum.

Then, the surface of the mold is sprayed with a first treatmentcomposition containing a second inorganic salt having a melting pointlower than that of the first inorganic salt and at least one liquidselected from water and a water-miscible organic solvent, or the mold isdipped in the first treatment composition, and the mold after eithertreatment is heated at a temperature at which the first inorganic saltis not fused but the second inorganic salt is fused, in anoxygen-containing atmosphere, for example, in an air atmosphere, wherebythe final mold is produced. At this time, the water-soluble organicpolymer in the mold is burned out (degreased).

The water-soluble first inorganic salt in the powder mixture and thesecond inorganic salt in the first treatment composition are used in,for example, the combinations described above in the third embodiment.

The first treatment composition is mainly in the form of a solution. Theconcentration of the second inorganic salt in the first treatmentcomposition is preferably 0.1% by weight or more from the viewpoint ofimproving strength of a mold by the second inorganic salt. The upperlimit of the concentration of the second inorganic salt in the firsttreatment composition is preferably the maximum concentration of thesecond inorganic salt that can be dissolved at room temperature in atleast one selected from a water-miscible organic solvent and water.Particularly, the concentration of the second inorganic salt in thefirst treatment composition is preferably 0.1 to 25% by weight. However,the first treatment composition consisting of the second inorganic saltcombined with the water-miscible organic solvent is sometimes in theform of a slurry in which the second inorganic salt is suspended withoutdissolution in the water-miscible organic solvent, depending on theirtypes.

In the case of the combination where the first inorganic salt is sodiumchloride and the second inorganic salt is potassium chloride, theheating is conducted preferably at a temperature of 400 to 750□ in anoxygen-containing atmosphere, for example in an air atmosphere.

In the sixth embodiment, the surface of the mold subjected to sprayingor the like with the first treatment solution and to heating may furtherbe sprayed with a second treatment composition containing a thirdinorganic salt having a melting point lower than that of the firstinorganic salt and at least one liquid selected from water and awater-miscible organic solvent, or the mold may be dipped in the secondtreatment composition, and the mold after either treatment may be heatedat a temperature at which the first inorganic salt is not fused but atleast one selected from the second and third inorganic salts is fused.

The second treatment composition is mainly in the form of a solution.The concentration of the third inorganic salt in the second treatmentcomposition is preferably in the range of 0.1 to 25% by weight. However,the second treatment composition consisting of the third inorganic saltcombined with the water-miscible organic solvent is sometimes in theform of a slurry in which the third inorganic salt is suspended withoutdissolution in the water-miscible organic solvent, depending on theirtypes.

The third inorganic salt is selected from materials of the same type as,or of a different type from, that of the second inorganic salt. When thethird inorganic salt is a material of a different type from the firstand second inorganic salts, the combination of the first, second andthird inorganic salts includes, for example, a combination wherein thefirst inorganic salt is sodium chloride (NaCl), the second inorganicsalt is potassium chloride (KCl), and the third inorganic salt is sodiumiodide (NaI) or calcium chloride (CaCl₂).

In dipping the mold into the second treatment composition, the secondtreatment composition on the surface of the mold is preferably 20 to 50μm in thickness.

The “fusion” of at least one inorganic salt selected from the secondinorganic salt and the third inorganic salt in the heating treatmentmeans that when the second and third inorganic salts are of the sametype, the inorganic salts are simply fused. When the third inorganicsalt is a material of a type different from the second inorganic salt,the term “fusion” means (A) mere fusion of the second inorganic saltand/or the third inorganic salt, (B) formation of a solid solution ofall of the second and third inorganic salts upon fusion, (C) formationof a solid solution of some of the second and third inorganic salts andsimultaneous formation of a composite salt between the remaininginorganic salts upon fusion, and (D) formation of a composite salt ofall the second and third inorganic salts upon fusion. By fusing at leastone inorganic salt selected from the second and third inorganic salts inthis way, it is possible to further improve the strength of the mold, inaddition to an improvement in the surface smoothness of the mold.

When the third inorganic salt in the second treatment composition ispotassium chloride (KCl) in a combination wherein the first inorganicsalt in the mold is for example sodium chloride (NaCl) and the secondinorganic salt in the first treatment composition subjected to sprayingor the like is potassium chloride (KCl), the heating is conductedpreferably at a temperature of 400 to 750□ in an oxygen-containingatmosphere, for example in an air atmosphere. In the case of acombination wherein the first inorganic salt is NaCl, the secondinorganic salt is KCl, and the third inorganic salt is NaI, the heatingis conducted preferably at a temperature of 400 to 700□ in anoxygen-containing atmosphere, for example in an air atmosphere.

According to the sixth embodiment described above, the following actionand effect are demonstrated.

1) A powder mixture containing a water-soluble first inorganic saltpowder and a water-soluble organic polymer powder is expanded in a planeon a table, and then the expanded powder mixture layer is sprayedselectively through a nozzle with a spray liquid containing at least oneselected from a water-miscible organic solvent and water, whereby thewater-soluble organic polymer powder as a component in the powdermixture is dissolved in water in the spray liquid, thus rendering thebonding strength among particles of the powder mixture in the sprayedregion higher than that among particles of the powder mixture in theunsprayed region. Such expanding of the powder mixture and selectivespraying of the spray liquid through a nozzle are repeated severaltimes, whereby a mold of a desired shape shaped by the sprayed regionscan be formed.

2) The water-soluble organic polymer powder incorporated into the powdermixture acts as a powder binder in the sprayed region, while the sprayliquid is a composition containing at least one selected from awater-miscible organic solvent and water and being free of a bindercomponent involved substantially in increasing viscosity, therebypreventing the clogging of a nozzle upon spraying the spray liquid ontothe expanded powder mixture layer (by printing with, for example, athree-dimensional printer having a nozzle). As a result, the sprayedregions (slice regions for forming a mold) faithful to the section datainto which the three-dimensional CAD data has been sliced can be formedin laminated mold manufacturing. In addition, there is no limitation,attributable to clogging, of the number of laminates in the laminatedmold thus produced, and thus a larger mold can be manufactured.

Particularly, by using a spray liquid containing a water-miscibleorganic solvent, it is possible to suppress or prevent the blurring anddiffusion of the sprayed region upon spraying the expanded powdermixture layer through a nozzle with the spray liquid, thus enablingformation of sprayed regions (slice regions for forming a mold) faithfulto the section data into which the three-dimensional CAD data has beensliced.

3) The surface of the mold after taking out is sprayed with a firsttreatment composition containing a water-soluble second inorganic salthaving a melting point lower than that of the first inorganic salt andat least one liquid selected from water and a water-miscible organicsolvent, or the mold is dipped in the first treatment composition, andby either treatment, the surface of the mold is coated with the secondinorganic salt in the first treatment composition. Thereafter, the moldis heated at a temperature at which the first inorganic salt is notfused but the second inorganic salt is fused, whereby the fused secondinorganic salt permeates into minute voids in the first inorganic saltpowder from the surface of the mold to make the mold dense. By fusingthe second inorganic salt coated on the surface of the mold, the surfacecan be smoothened. As a result, a mold having a glossier surface thanthe mold just after removal can be obtained.

4) In demolding after casting with the mold as a core, the mold iscomposed mainly of the water-soluble first and second inorganic saltsand can thus be easily removed with water.

According to the sixth embodiment, there can be provided a method formanufacturing a mold, wherein a mold having a smooth and glossy surfaceand practically durable strength and being faithful to three-dimensionalCAD data can be produced reproducibly without clogging a nozzle inspraying the spray liquid and can be easily removed.

According to the sixth embodiment, the water-soluble organic polymer isburned out by heating the mold at a temperature at which the secondinorganic salt is fused, and thus the resulting mold has a slightlylower strength than that of the mold having the remaining water-solubleorganic polymer according to the first embodiment, but has the followingeffect.

That is, when such a mold wherein the water-soluble organic polymer hasbeen burned out by the heating is applied to a core where a molten metalis cast at a temperature higher than the temperature at which thewater-soluble organic polymer is burned out, for example, to a core foraluminum die-casting, the gas generation attributable to thewater-soluble organic polymer can be prevented, as described in thethird embodiment. As a result, bubble generation, etc. in thedie-casting product, accompanying the gas generation, can be prevented,and the die-casting product having a desired strength can be obtained.

The surface of the mold after the heating is sprayed with a secondtreatment composition containing a third inorganic salt having a meltingpoint lower than that of the first inorganic salt and at least oneliquid selected from water and a water-miscible organic solvent, or themold is dipped in the second treatment composition, and the mold aftereither treatment is heated at a temperature at which the first inorganicsalt is not fused but at least one inorganic salt selected from thesecond and third inorganic salts is fused. By coating the surface of themold with the third inorganic salt in the second treatment compositionand subsequent heat treatment, the surface of the mold is smoothened bycoating it with a thin film of the third inorganic salt. In addition,the mold is densified by fusing at least one of the second and thirdinorganic salts. Particularly, both the second and third inorganic saltscan be fused to form a mutual solid solution or to form a compositesalt, thus further densifying the mold and improving the strengththereof. As a result, a high-strength mold having a smoother andglossier surface can be obtained.

When such a mold having a smooth surface is applied to a core foraluminum die-casting, gas generation attributable to the water-solubleorganic polymer can be avoided, thereby preventing bubble generation inthe die-casting product, as described above, and the contact surface ofthe die-casting product with the mold (core) can be made glossy, andthus a high-grade die-casting product having a desired strength can beobtained.

Hereinafter, the present invention will be described in more detail withreference to Examples.

EXAMPLE 1

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 0.5 wt % polyvinyl pyrrolidone (PVP) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, ethanol (EtOH) and water were mixed in a weight ratio of 1:1and compounded with a very small amount of a blue dye (edible blueNo. 1) to prepare a spray liquid.

First, the powder mixture was expanded in a plane on a table, to form apowder mixture layer of about 0.18 mm in thickness. Using athree-dimensional printer having a nozzle, this mixed powder was thenprinted (sprayed through the nozzle) selectively with the above sprayliquid at room temperature (20□), on the basis of the bottom (data onthe lowest section) out of section data obtained by converting data on arectangular mold of 5.2 mm in width, 14.1 mm in length and 5.2 mm inheight prepared using a three-dimensional CAD, into data on sectionseach sliced in a thickness of about 0.18 mm. At this time, the bondingstrength among particles of the powder mixture in the sprayed region(colored blue) of the powder mixture layer was made higher than thatamong particles of the powder mixture in the unsprayed region.

Then, the powder mixture was further expanded on the powder mixturelayer after spraying, to form a powder mixture layer. Using thethree-dimensional printer having a nozzle, this expanded powder mixturelayer was then printed selectively (sprayed through the nozzle) with thespray liquid on the basis of the section data (data on an upper sectionnext to the bottom). At this time, the bonding strength among particlesof the powder mixture in the sprayed region (colored blue) in the powdermixture layer was made higher than that among particles of the powdermixture in the unsprayed region. Simultaneously, this sprayed region wasbound to the lower sprayed region.

Then, such formation of a powder mixture layer and subsequent selectiveprinting (nozzle spraying) of this powder mixture layer with the sprayliquid by the three-dimensional printer were conducted repeatedly 31times. By such laminated mold manufacturing, a rectangular mold shapedby the sprayed regions formed by spraying the spray liquid 31 times wasformed. Subsequently, the mold was dried at 100□ for 30 minutes and thena rectangular mold was manufactured by removing the powder mixture inthe unsprayed region and taking out the mold.

In the same manner, 4 additional molds were produced, and 5 molds wereobtained in total.

EXAMPLE 2

Five rectangular molds were prepared in the same manner as in Example 1except that a mixture prepared by compounding sodium chloride (NaCl)powder having the same average particle size as in Example 1 with 1 wt %PVP powder having the same average particle size as in Example 1 wasused as the powder mixture. The same spray liquid as in Example 1, thatis, a mixture prepared by mixing EtOH with water in a weight ratio of1:1 and adding a very small amount of a blue dye (edible blue No. 1)thereto was used.

EXAMPLE 3

Five rectangular molds were prepared in the same manner as in Example 1except that a mixture prepared by compounding sodium chloride (NaCl)powder having the same average particle size as in Example 1 with 5.0 wt% PVP powder having the same average particle size as in Example 1 wasused as the powder mixture. The same spray liquid as in Example 1, thatis, a mixture prepared by mixing EtOH with water in a weight ratio of1:1 and adding a very small amount of a blue dye (edible blue No. 1)thereto was used.

EXAMPLE 4

Five rectangular molds were prepared in the same manner as in Example 1except that a mixture prepared by compounding sodium chloride (NaCl)powder having the same average particle size as in Example 1 with 10 wt% PVP powder having the same average particle size as in Example 1 wasused as the powder mixture. The same spray liquid as in Example 1, thatis, a mixture prepared by mixing EtOH with water in a weight ratio of1:1 and adding a very small amount of a blue dye (edible blue No. 1)thereto was used.

EXAMPLE 5

Five rectangular molds were prepared in the same manner as in Example 1except that a mixture prepared by compounding sodium chloride (NaCl)powder having the same average particle size as in Example 1 with 1 wt %PVP powder having the same average particle size as in Example 1 wasused as the powder mixture, and a mixture prepared by mixing EtOH withwater in a weight ratio of 1:2 and adding a very small amount of a bluedye (edible blue No. 1) thereto was used as the spray liquid.

EXAMPLE 6

Five rectangular molds were prepared in the same manner as in Example 1except that a mixture prepared by compounding sodium chloride (NaCl)powder having the same average particle size as in Example 1 with 10 wt% PVP powder having the same average particle size as in Example 1 wasused as the powder mixture, and a mixture prepared by mixing EtOH withwater in a weight ratio of 1:2 and adding a very small amount of a bluedye (edible blue No. 1) thereto was used as the spray liquid.

EXAMPLE 7

Five rectangular molds were prepared in the same manner as in Example 1except that a mixture prepared by compounding sodium chloride (NaCl)powder having the same average particle size as in Example 1 with 1 wt %PVP powder having the same average particle size as in Example 1 wasused as the powder mixture, and water was used as the spray liquid.

COMPARATIVE EXAMPLE 1

Five rectangular molds were prepared in the same manner as in Example 1except that only sodium chloride (NaCl) powder having the same averageparticle size as in Example 1 was used as the powder to be expanded. Thesame spray liquid as in Example 1, that is, a mixture prepared by mixingEtOH with water in a weight ratio of 1:1 and adding a very small amountof a blue dye (edible blue No. 1) thereto was used.

COMPARATIVE EXAMPLE 2

Five rectangular molds were prepared in the same manner as in Example 1except that only sodium chloride (NaCl) powder having the same averageparticle size as in Example 1 was used as the powder to be expanded, andwater containing a very small amount of a blue dye (edible blue No. 1)was used as the spray liquid.

The resulting rectangular molds in Examples 1 to 7 and ComparativeExamples 1 and 2 were examined for their strength under the followingconditions.

Measuring instrument: Particle strength measuring instrument (tradename: Grano, manufactured by Okada Seiko Co., Ltd.)

Measurement Conditions

Tip diameter of a chip pressed against the central part of the mold: 3mm

Measurement speed of a chip: 100 μm/sec

Measurement range: 2 kg to 20 kg.

Measurement position: The tip of a chip was arranged to be positioned onthe central part of the mold.

Evaluation method: The 5 molds were measured for their strength, and themean strength of 3 molds, excluding 2 molds with the maximum and minimumvalues respectively, was determined.

The results in this strength test are shown in FIG. 2.

From FIG. 2, it can be seen that the molds in Examples 1 to 7 havehigher strength than the mold in Comparative Example 1 wherein only NaClpowder not containing PVP powder was used as the expanded powder and themold in Comparative Example 2 wherein only NaCl powder not containingPVP powder was used as the expanded powder and water was used as thespray liquid. It can be seen that particularly, the molds in Examples 2to 6 wherein a powder mixture containing 1 wt % or more PVP powder wasused as the expanded powder and a mixture of ethanol and water was usedas the spray liquid, have a high strength of about 240 g/mm² or more.

With respect to the resulting rectangular molds in Examples 1 to 7 andComparative Examples 1 and 2, their appearance was visually observed.

As a result, it was confirmed that the molds in Examples 1 to 7 have aclearer edge line than in the mold in Comparative Example 1 wherein onlysodium chloride powder not containing PVP powder was used as theexpanded powder and the mold in Comparative Example 2 wherein onlysodium chloride powder not containing PVP powder was used as theexpanded powder and water was used as the spray liquid. It was confirmedthat particularly, the molds in Examples 2 to 6 wherein a powder mixturecontaining 1 wt % or more PVP powder was used as the expanded powder anda mixture of ethanol and water was used as the spray liquid, have aclearer edge line.

EXAMPLE 8

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 1.0 wt % polyvinyl pyrrolidone (PVP) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, ethanol and water were mixed in a weight ratio of 1:1 andcompounded with a very small amount of a blue dye (edible blue No. 1) toprepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

EXAMPLE 9

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 1.0 wt % polyvinyl pyrrolidone (PVP) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, acetone and water were mixed in a weight ratio of 1:1 andcompounded with a very small amount of a blue dye (edible blue No. 1) toprepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

EXAMPLE 10

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 1.0 wt % polyvinyl alcohol (PVA) powder having anaverage particle size of 117 μm to prepare a powder mixture.

Separately, ethanol and water were mixed in a weight ratio of 1:1 andcompounded with a very small amount of a blue dye (edible blue No. 1) toprepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

EXAMPLE 11

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 1.0 wt % carboxymethyl cellulose (CMC) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, ethanol and water were mixed in a weight ratio of 1:1 andcompounded with a very small amount of a blue dye (edible blue No. 1) toprepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

EXAMPLE 12

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 1.0 wt % hydroxypropyl cellulose (HPC) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, ethanol and water were mixed in a weight ratio of 1:1 andcompounded with a very small amount of a blue dye (edible blue No. 1) toprepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

With respect to the resulting rectangular molds in Examples 8 to 12,their appearance was visually observed. As a result, it was confirmedthat the molds in Examples 8 to 12 have a clearer edge line than in themold in Comparative Example 1 wherein only sodium chloride powder notcontaining a water-soluble organic polymer resin powder such as PVPpowder was used as the expanded powder and the mold in ComparativeExample 2 wherein only sodium chloride powder not containing awater-soluble organic polymer resin powder was used as the expandedpowder and water was used as the spray liquid.

The resulting rectangular molds in Examples 8 to 12 were examined in thestrength test under the same conditions as described above, and the meanstrength of 3 molds, excluding 2 molds with the maximum and minimumvalues respectively, was determined. The results are shown in Table 1below.

TABLE 1 Powder mixture Inorganic Organic Organic solvent Strength saltpolymer in spray liquid (g/mm²) Example 8 NaCl PVP Ethanol 240 Example 9NaCl PVP Acetone 255 Example 10 NaCl PVA Ethanol 148 Example 11 NaCl CMCEthanol 155 Example 12 NaCl HPC Ethanol 180

From Table 1 above, it can be seen that the molds in Examples 8 to 12have higher strength than the mold in Comparative Example 1 wherein onlysodium chloride powder not containing a water-soluble organic polymerresin powder such as PVP powder was used as the expanded powder shown inFIG. 1 and the mold in Comparative Example 2 wherein only sodiumchloride powder not containing a water-soluble organic polymer resinpowder was used as the expanded powder and water was used as the sprayliquid. It can be seen that, particularly, the molds in Examples 8 and 9wherein PVP powder was used as the water-soluble organic polymer resinpowder in the expanded powder have a higher strength than in the moldsin Examples 12 to 14 wherein a water-soluble organic polymer resinpowder such as CMC powder was used.

EXAMPLE 13-1

A mixture (NaCl:MgSO₄ weight ratio=95:5) of sodium chloride (NaCl)powder having an average particle size of 63 μm and magnesium sulfate(MgSO₄) powder having an average particle size of 50 μm was compoundedwith 2.0 wt % polyvinyl pyrrolidone (PVP) powder having an averageparticle size of 117 μm to prepare a powder mixture.

Separately, ethanol (EtOH) and water were mixed in a weight ratio of 1:1and compounded with a very small amount of a blue dye (edible blueNo. 1) to prepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

Then, the resulting molds were subjected to heat treatment at 700□ for30 minutes in air to prepare final molds.

EXAMPLE 13-2

Sodium iodide (NaI) was dissolved in an amount of 10% by weight inethanol to prepare a dipping treatment solution. The respective moldsobtained in Example 13-1 were dipped in this dipping treatment solutionfor 2 minutes, then removed from the dipping treatment solution, andsubjected respectively to heat treatment at 700□ for 30 minutes in air.

With respect to the resulting rectangular molds in Examples 13-1 and13-2, their appearance was visually observed. As a result, it wasconfirmed that the molds in both the Examples have a sharp shape with aclearer edge line than in the molds in Examples 8 to 12.

With respect to the resulting rectangular molds in Examples 13-1 and13-2, their surface state was visually observed. As a result, it wasconfirmed that the molds in Example 13-1 have a slightly rough surface,but the molds in Examples 13-2 have a smooth surface.

The resulting rectangular molds in Examples 13-1 and 13-2 were examinedin the strength test under the same conditions as described above, andthe mean strength of 3 molds, excluding 2 molds with the maximum andminimum values respectively, was determined. As a result, the molds inExample 13-1 had a strength of 190 g/mm², and the molds in Example 13-2had a strength of 220 g/mm².

When such a mold in Example 13-1 is applied to a core where a moltenmetal is cast at a temperature higher than the temperature at which PVPis burned out, for example, to a core for aluminum die-casting, gasgeneration attributable to PVP can be avoided, thereby preventing bubblegeneration, etc. in the die-casting product, and thus a die-castingproduct having desired strength can be obtained.

When such a mold having a smooth surface in Example 13-2 is applied to acore for aluminum die-casting, gas generation attributable to PVP can beavoided, thereby preventing bubble generation, etc. in the die-castingproduct, and the contact surface of a die-casting product with the mold(core) can be made glossy, and thus a high-grade die-casting producthaving desired strength can be obtained.

EXAMPLE 14-1

A mixture (NaCl:KCl weight ratio=95:5) of sodium chloride (NaCl) powderhaving an average particle size of 63 μm and potassium chloride (KCl)powder having an average particle size of 63 μm was compounded with 2.0wt % polyvinyl pyrrolidone (PVP) powder having an average particle sizeof 117 μm to prepare a powder mixture.

Separately, ethanol (EtOH) and water were mixed in a weight ratio of 1:1and compounded with a very small amount of a blue dye (edible blueNo. 1) to prepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

Then, the resulting molds were subjected to heating at 700□ for 30minutes in air to manufacture final molds.

EXAMPLE 14-2

Sodium iodide (NaI) was dissolved in an amount of 10% by weight inethanol to prepare a dipping treatment solution. The respective moldsobtained in Example 14-1 were dipped in this dipping treatment solutionfor 2 minutes, then taken out from the dipping treatment solution, andsubjected respectively to heating at 700□ for 30 minutes in air.

EXAMPLE 15-1

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 2.0 wt % polyvinyl pyrrolidone (PVP) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, potassium chloride (KCl) was dissolved in an amount of 5 wt% in a mixture prepared by mixing ethanol (EtOH) with water in a weightratio of 1:1, followed by adding a very small amount of a blue dye(edible blue No. 1) thereto, to prepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

Then, the resulting molds were subjected to heating at 700□ for 30minutes in air to manufacture final molds.

EXAMPLE 15-2

Sodium iodide (NaI) was dissolved in an amount of 10% by weight inethanol to prepare a dipping treatment solution. The respective moldsobtained in Example 15-1 were dipped in this dipping treatment solutionfor 2 minutes, then taken out from the dipping treatment solution, andsubjected respectively to heating at 700□ for 30 minutes in air.

EXAMPLE 16-1

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 2.0 wt % polyvinyl pyrrolidone (PVP) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, sodium iodide (NaI) was dissolved in an amount of 10% byweight in ethanol (EtOH), followed by adding a very small amount of ablue dye (edible blue No. 1) thereto, to prepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

Then, the resulting molds were subjected to heating at 700□ for 30minutes in air to manufacture final molds.

EXAMPLE 16-2

Calcium chloride (CaCl₂) was dissolved in an amount of 5% by weight inethanol to prepare a dipping treatment solution. The respective moldsobtained in Example 16-1 were dipped in this dipping treatment solutionfor 2 minutes, then taken out from the dipping treatment solution, andsubjected respectively to heating at 700□ for 30 minutes in the air.

EXAMPLE 17-1

Magnesium sulfate (MgSO₄) powder having an average particle size of 50μm was compounded with 2.0 wt % polyvinyl pyrrolidone (PVP) powderhaving an average particle size of 117 μm to prepare a powder mixture.

Separately, calcium chloride (CaCl₂) was dissolved in an amount of 5% byweight in ethanol (EtOH), followed by adding a very small amount of ablue dye (edible blue No. 1) thereto, to prepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

Then, the resulting molds were subjected to heating at 700□ for 30minutes in air to manufacture final molds.

EXAMPLE 17-2

Sodium iodide (NaI) was dissolved in an amount of 5% by weight inethanol to prepare a dipping treatment solution. The respective moldsobtained in Example 17-1 were dipped in this dipping treatment solutionfor 2 minutes, then taken out from the dipping treatment solution, andsubjected respectively to heating at 700□ for 30 minutes in air.

EXAMPLE 18-1

Sodium chloride (NaCl) powder having an average particle size of 63 μmwas compounded with 2.0 wt % polyvinyl pyrrolidone (PVP) powder havingan average particle size of 117 μm to prepare a powder mixture.

Separately, ethanol (EtOH) and water were mixed in a weight ratio of 1:1and compounded with a very small amount of a blue dye (edible blueNo. 1) to prepare a spray liquid.

The powder mixture and the spray liquid were used to produce 5rectangular molds in the same manner as in Example 1.

A spray treatment solution was previously prepared by dissolvingpotassium chloride (KCl) in an amount of 5% by weight in a mixturehaving ethanol (EtOH) and water mixed in a weight ratio of 1:1, and thisspray treatment solution was sprayed onto the surfaces of the respectivemolds which were then subjected to heating at 700□ for 30 minutes in airto manufacture final molds.

EXAMPLE 18-2

Sodium iodide (NaI) was dissolved in an amount of 10% by weight inethanol to prepare a dipping treatment solution. The respective moldsobtained in Example 18-1 were dipped in this dipping treatment solutionfor 2 minutes, then taken out from the dipping treatment solution, andsubjected respectively to heating at 700□ for 30 minutes in air.

With respect to the resulting rectangular molds in Examples 14-1, 14-2,15-1, 15-2, 16-1, 16-2, 17-1, 17-2, 18-1 and 18-2, their appearance wasvisually observed. As a result, it was confirmed that the molds in anyof the Examples have a clearer edge line than in the mold in ComparativeExample 1 wherein only sodium chloride powder not containing awater-soluble organic polymer resin powder such as PVP powder was usedas the expanded powder and the mold in Comparative Example 2 whereinonly sodium chloride powder not containing a water-soluble organicpolymer resin powder was used as the expanded powder and water was usedas the spray liquid.

With respect to the resulting rectangular molds in Examples 14-1, 14-2,15-1, 15-2, 16-1, 16-2, 17-1, 17-2, 18-1 and 18-2, their surface statewas visually observed. The results are shown in Table 2 below.

With respect to the resulting rectangular molds in Examples 14-1, 14-2,15-1, 15-2, 16-1, 16-2, 17-1, 17-2, 18-1 and 18-2, the strength test wascarried out under the same conditions as described above, and the meanstrength of 3 molds, excluding 2 molds with the maximum and minimumvalues respectively, was determined. The results are shown in Table 2below.

TABLE 2 Powder mixture Spray liquid Inorganic salt in Inorganic salt inInorganic Organic Inorganic spray treatment dipping treatment SurfaceStrength salt polymer salt Solution solution solution state (g/mm²)Example NaCl/KCl PVP — EtOH — — Slightly rough 130 14-1 50 wt % ExampleNaCl/KCl PVP — EtOH — Nal Smooth 215 14-2 50 wt % Example NaCl PVP KClEtOH — — Slightly rough 100 15-1 50 wt % Example NaCl PVP KCl EtOH — NalSmooth 170 15-2 50 wt % Example NaCl PVP NaI EtOH — — Slightly rough 20016-1 100 wt % Example NaCl PVP NaI EtOH — CaCl₂ Smooth 230 16-2 100 wt %Example MgSO₄ PVP CaCl₂ EtOH — — Slightly rough 85 17-1 100 wt % ExampleMgSO₄ PVP CaCl₂ EtOH — NaI Smooth 134 17-2 100 wt % Example NaCl PVP —EtOH KCl — Slightly rough 99 18-1 50 wt % Example NaCl PVP — EtOH KClNaI Smooth 168 18-2 50 wt %

As is evident from Table 2 above, the molds in Examples 14-1, 15-1,16-1, 17-1 and 18-1, have burned out PVP due to the heat treatment andare thus slightly inferior in strength to the molds in Examples 8 to 12(with an organic polymer such as PVP remaining therein), but have apractically durable strength. In addition, when such mold is applied toa core where a molten metal is cast at a temperature higher than thetemperature at which PVP is burned out, for example, to a core foraluminum die-casting, the gas generation attributable to PVP can beavoided, thereby preventing bubble generation, etc. in the die-castingproduct, and thus a die-casting product having a desired strength can beobtained.

It can also be seen that the molds in Examples 14-2, 15-2, 16-2, 17-2and 18-2 wherein the molds have been subjected to dipping in a dippingtreatment solution having an inorganic salt dissolved therein andfurther to heating, have improved not only surface smoothness but alsostrength, as compared with the molds in Examples 14-1, 15-1, 16-1, 17-1and 18-1 wherein the molds are not subjected to such treatment. Whensuch a mold having a smooth surface is applied to a core for aluminumdie-casting, gas generation attributable to PVP can be avoided, therebypreventing bubble generation, etc. in the die-casting product, and thecontact surface of the die-casting product with the mold (core) can bemade glossy, and thus a high-grade die-casting product having a desiredstrength can be obtained.

INDUSTRIAL APPLICABILITY

According to the present invention, there can be provided a method formanufacturing a mold, wherein a mold having a complicated shape andsufficient strength useful for a core, etc. used in casting can beproduced easily, safely and accurately.

According to the present invention, there can further be provided amethod for manufacturing a mold, wherein a mold having a complicatedshape, high strength and a smooth surface useful for a core, etc. usedin casting can be produced easily, safely and accurately.

1. A method for manufacturing a mold, comprising steps of: (a) expandinga powder mixture containing a water-soluble inorganic salt powder and awater-soluble organic polymer powder in a plane on a table and thenselectively spraying a spray liquid containing at least one selectedfrom a water-miscible organic solvent and water through a nozzle on thispowder mixture layer to make a bonding strength among particles of thepowder mixture in a sprayed region higher than that among particles ofthe powder mixture in an unsprayed region; (b) further expanding thepowder mixture on the powder mixture layer after spraying and thenselectively spraying the spray liquid through the nozzle on this powdermixture layer; (c) repeatedly conducting the step (b) over several timesto form a mold shaped by the sprayed regions; and (d) taking out themold.
 2. The method for manufacturing a mold according to claim 1,wherein the water-soluble inorganic salt is an alkali metal salt or analkaline earth metal salt.
 3. The method for manufacturing a moldaccording to claim 1, wherein the water-soluble organic polymer is atleast one selected from polyvinyl pyrrolidone, polyvinyl alcohol,polyethylene glycol and a cellulose derivative.
 4. The method formanufacturing a mold according to claim 1, wherein the water-miscibleorganic solvent is a lower alcohol or a ketone.
 5. The method formanufacturing a mold according to claim 1, wherein drying treatment isfurther conducted after the step (c) and before the step (d).
 6. Amethod for manufacturing a mold, comprising steps of: (a) expanding apowder mixture containing a water-soluble first inorganic salt powder, asecond inorganic salt powder consisting of magnesium sulfate, and awater-soluble organic polymer powder in a plane on a table and thenselectively spraying a spray liquid containing at least one selectedfrom a water-miscible organic solvent and water through a nozzle on thispowder mixture layer to make a bonding strength among particles of thepowder mixture in a sprayed region higher than that among particles ofthe powder mixture in an unsprayed region; (b) further expanding thepowder mixture on the powder mixture layer after spraying and thenselectively spraying the spray liquid through the nozzle on this powdermixture layer; (c) repeatedly conducting the step (b) over several timesto form a mold shaped by the sprayed regions; (d) taking out the mold;and (e) heating the mold at a temperature between the temperature atwhich the water-soluble organic polymer is burned out and thetemperature at which the first and second inorganic salts are not fused.7. The method for manufacturing a mold according to claim 6, wherein thefirst inorganic salt is sodium chloride.
 8. The method for manufacturinga mold according to claim 6, wherein the water-soluble organic polymeris at least one member selected from polyvinyl pyrrolidone, polyvinylalcohol, polyethylene glycol and a cellulose derivative.
 9. The methodfor manufacturing a mold according to claim 6, wherein thewater-miscible organic solvent is a lower alcohol or a ketone.
 10. Themethod for manufacturing a mold according to claim 6, wherein dryingtreatment is further conducted after the step (c) and before the step(d).
 11. The method for manufacturing a mold according to claim 6,wherein a surface of the mold after the step (e) is further sprayed witha treatment composition containing a third inorganic salt having amelting point lower than those of the first and second inorganic saltsand at least one liquid selected from water and a water-miscible organicsolvent, or the mold is dipped in the treatment composition, and themold after either treatment is heated at a temperature at which thefirst and second inorganic salts are not fused but the third inorganicsalt is fused.
 12. A method for manufacturing a mold, comprising stepsof: (a) expanding a powder mixture containing a water-soluble firstinorganic salt water, a water-soluble second inorganic salt powderhaving a melting point lower than that of the first inorganic salt, anda water-soluble organic polymer powder in a plane on a table and thenselectively spraying a spray liquid containing at least one selectedfrom a water-miscible organic solvent and water through a nozzle on thispowder mixture layer to make a bonding strength among particles of thepowder mixture in a sprayed region higher than that among particles ofthe powder mixture in an unsprayed region; (b) further expanding thepowder mixture on the powder mixture layer after spraying and thenselectively spraying the spray liquid through the nozzle on this powdermixture layer; (c) repeatedly conducting the step (b) over several timesto form a mold shaped by the sprayed regions; (d) taking out the mold;and (e) heating the mold at a temperature at which the first inorganicsalt is not fused but the second inorganic salt is fused.
 13. The methodfor manufacturing a mold according to claim 12, wherein the firstinorganic salt is sodium chloride, and the second inorganic salt ispotassium chloride.
 14. The method for manufacturing a mold according toclaim 12, wherein the water-soluble organic polymer is at least onemember selected from polyvinyl pyrrolidone, polyvinyl alcohol,polyethylene glycol and a cellulose derivative.
 15. The method formanufacturing a mold according to claim 12, wherein the water-miscibleorganic solvent is a lower alcohol or a ketone.
 16. The method formanufacturing a mold according to claim 12, wherein drying treatment isfurther conducted after the step (c) and before the step (d).
 17. Themethod for manufacturing a mold according to claim 12, wherein a surfaceof the mold after the step (e) is further sprayed with a treatmentcomposition containing a third inorganic salt having a melting pointlower than that of the first inorganic salt and at least one liquidselected from water and a water-miscible organic solvent, or the mold isdipped in the treatment composition, and the mold after either treatmentis heat-treated at a temperature at which the first inorganic salt isnot fused but at least one inorganic salt selected from the second andthird inorganic salts is fused.
 18. A method for manufacturing a mold,comprising steps of: (a) expanding a powder mixture containing awater-soluble first inorganic salt powder and a water-soluble organicpolymer powder in a plane on a table and then selectively spraying aspray liquid containing a second inorganic salt having a melting pointlower than that of the first inorganic salt, and either water or waterand a water-miscible organic solvent through a nozzle on this powdermixture layer to make a bonding strength among particles of the powdermixture in a sprayed region higher than that among particles of thepowder mixture in an unsprayed region; (b) further expanding the powdermixture on the powder mixture layer after spraying and then selectivelyspraying the spray liquid through the nozzle on this powder mixturelayer; (c) repeatedly conducting the step (b) over several times to forma mold shaped by the sprayed regions; (d) taking out the mold; and (e)heating the mold at a temperature at which the first inorganic salt isnot fused but the second inorganic salt is fused.
 19. The method formanufacturing a mold according to claim 18, wherein the first inorganicsalt is sodium chloride, and the second inorganic salt is potassiumchloride.
 20. The method for manufacturing a mold according to claim 18,wherein the water-soluble organic polymer is at least one memberselected from polyvinyl pyrrolidone, polyvinyl alcohol, polyethyleneglycol and a cellulose derivative.
 21. The method for manufacturing amold according to claim 18, wherein the water-miscible organic solventis a lower alcohol or a ketone.
 22. The method for manufacturing a moldaccording to claim 18, wherein drying treatment is further conductedafter the step (c) and before the step (d).
 23. The method formanufacturing a mold according to claim 18, wherein a surface of themold after the step (e) is further sprayed with a treatment compositioncontaining a third inorganic salt having a melting point lower than thatof the first inorganic salt and at least one liquid selected from waterand a water-miscible organic solvent, or the mold is dipped in thetreatment composition, and the mold after either treatment isheat-treated at a temperature at which the first inorganic salt is notfused but at least one inorganic salt selected from the second and thirdinorganic salts is fused.
 24. A method for manufacturing a mold,comprising steps of: (a) expanding a powder mixture containing awater-soluble first inorganic salt powder and an organic polymer powderin a plane on a table and then selectively spraying a spray liquidcontaining at least one second inorganic salt having a melting pointlower than that of the first inorganic salt and selected from calciumchloride and sodium iodide and at least one organic solvent selectedfrom a lower alcohol and a ketone through a nozzle on this powdermixture layer to make a bonding strength among particles of the powdermixture in a sprayed region higher than that among particles of thepowder mixture in an unsprayed region; (b) further expanding the powdermixture on the powder mixture layer after spraying and then selectivelyspraying the spray liquid through the nozzle on this powder mixturelayer; (c) repeatedly conducting the step (b) over several times to forma mold shaped by the sprayed regions; (d) taking out the mold; and (e)heating the mold at a temperature at which the first inorganic salt isnot fused but the second inorganic salt is fused.
 25. The method formanufacturing a mold according to claim 24, wherein the first inorganicsalt is sodium chloride, sodium bromide, potassium chloride, potassiumbromide, magnesium chloride or magnesium sulfate.
 26. The method formanufacturing a mold according to claim 24, wherein the organic polymeris at least one member selected from polyvinyl pyrrolidone, polyvinylalcohol, polyethylene glycol and a cellulose derivative.
 27. The methodfor manufacturing a mold according to claim 24, wherein drying treatmentis further conducted after the step (c) and before the step (d).
 28. Themethod for manufacturing a mold according to claim 24, wherein a surfaceof the mold after the step (e) is further sprayed with a treatmentcomposition containing a third inorganic salt having a melting pointlower than that of the first inorganic salt and at least one liquidselected from water and a water-miscible organic solvent, or the mold isdipped in the treatment composition, and the mold after either treatmentis heat-treated at a temperature at which the first inorganic salt isnot fused but at least one inorganic salt selected from the second andthird inorganic salts is fused.
 29. A method for manufacturing a mold,comprising steps of: (a) expanding a powder mixture containing awater-soluble first inorganic salt powder and a water-soluble organicpolymer powder in a plane on a table and then selectively spraying aspray liquid containing at least one selected from a water-miscibleorganic solvent and water through a nozzle on this powder mixture layerto make a bonding strength among particles of the powder mixture in asprayed region higher than that among particles of the powder mixture inan unsprayed region; (b) further expanding the powder mixture on thepowder mixture layer after spraying and then selectively spraying thespray liquid through the nozzle on this powder mixture layer; (c)repeatedly conducting the step (b) over several times to form a moldshaped by the sprayed regions; (d) taking out the mold; and (e) furtherspraying a treatment composition containing a water-soluble secondinorganic salt having a melting point lower than that of the firstinorganic salt and at least one liquid selected from water and awater-miscible organic solvent on a surface of the mold, or dipping themold in the treatment composition, and then heating the mold aftereither treatment at a temperature at which the first inorganic salt isnot fused but the second inorganic salt is fused.
 30. The method formanufacturing a mold according to claim 29, wherein the first inorganicsalt is sodium chloride, and the second inorganic salt is potassiumchloride.
 31. The method for manufacturing a mold according to claim 29,wherein the water-soluble organic polymer is at least one memberselected from polyvinyl pyrrolidone, polyvinyl alcohol, polyethyleneglycol and a cellulose derivative.
 32. The method for manufacturing amold according to claim 29, wherein the water-miscible organic solventis a lower alcohol or a ketone.
 33. The method for manufacturing a moldaccording to claim 29, wherein drying treatment is further conductedafter the step (c) and before the step (d).
 34. The method formanufacturing a mold according to claim 29, wherein the surface of themold after the step (e) is further sprayed with a treatment compositioncontaining a third inorganic salt having a melting point lower than thatof the first inorganic salt and at least one liquid selected from waterand a water-miscible organic solvent, or the mold is dipped in thetreatment composition, and the mold after either treatment isheat-treated at a temperature at which the first inorganic salt is notfused but at least one inorganic salt selected from the second and thirdinorganic salts is fused.